diff --git a/app/game-frontend/e2e/board.spec.ts b/app/game-frontend/e2e/board.spec.ts
index 0a027abf..183f2aa3 100644
--- a/app/game-frontend/e2e/board.spec.ts
+++ b/app/game-frontend/e2e/board.spec.ts
@@ -274,6 +274,8 @@ test('matches the ref meeting-room geometry, typography, textures, and controls'
         'src',
         'https://sam-image.hided.net/icons/22.jpg'
     );
+    await expect(page.locator('.article-header .date')).toHaveText('07-26 19:20');
+    await expect(page.locator('.comment-row .date')).toHaveText('07-26 19:25');
     if (artifactRoot) {
         await page.screenshot({
             path: resolve(artifactRoot, 'board-core-desktop.png'),
@@ -392,7 +394,9 @@ test('uses the ref 500px responsive form widths', async ({ page }) => {
     await expect(page.getByRole('heading', { name: '기밀실' })).toBeVisible();
 });
 
-test('retains article and comment input after a failed mutation, then reloads after success', async ({ page }, testInfo) => {
+test('retains article and comment input after a failed mutation, then reloads after success', async ({
+    page,
+}, testInfo) => {
     const state: BoardFixture = {
         permission: 2,
         canMeeting: true,
@@ -408,7 +412,9 @@ test('retains article and comment input after a failed mutation, then reloads af
     await page.locator('#board-title').fill('새 제목');
     await page.locator('#board-content').fill('새 내용');
     await page.locator('#submitArticle').click();
-    const articleToast = page.getByTestId('game-toast').filter({ hasText: '게시물 등록에 실패했습니다: 접속 제한입니다.' });
+    const articleToast = page
+        .getByTestId('game-toast')
+        .filter({ hasText: '게시물 등록에 실패했습니다: 접속 제한입니다.' });
     await expect(articleToast).toHaveAttribute('data-feedback-kind', 'error');
     await expect(articleToast).toHaveAttribute('role', 'alert');
     await expect(page.locator('#board-title')).toHaveValue('새 제목');
@@ -416,7 +422,13 @@ test('retains article and comment input after a failed mutation, then reloads af
 
     const desktopToastGeometry = await articleToast.evaluate((element) => {
         const rect = element.getBoundingClientRect();
-        return { left: rect.left, right: rect.right, top: rect.top, width: rect.width, viewportWidth: window.innerWidth };
+        return {
+            left: rect.left,
+            right: rect.right,
+            top: rect.top,
+            width: rect.width,
+            viewportWidth: window.innerWidth,
+        };
     });
     expect(desktopToastGeometry.left).toBeGreaterThanOrEqual(0);
     expect(desktopToastGeometry.right).toBeLessThanOrEqual(desktopToastGeometry.viewportWidth);
@@ -435,10 +447,14 @@ test('retains article and comment input after a failed mutation, then reloads af
     await page.setViewportSize({ width: 390, height: 844 });
     const documentWidthBeforeToast = await page.evaluate(() => document.documentElement.scrollWidth);
     await commentInput.press('Enter');
-    const commentToast = page.getByTestId('game-toast').filter({ hasText: '댓글 등록에 실패했습니다: 접속 제한입니다.' });
+    const commentToast = page
+        .getByTestId('game-toast')
+        .filter({ hasText: '댓글 등록에 실패했습니다: 접속 제한입니다.' });
     await expect(commentToast).toBeVisible();
     await expect
-        .poll(async () => commentToast.evaluate((element) => window.innerHeight - element.getBoundingClientRect().bottom))
+        .poll(async () =>
+            commentToast.evaluate((element) => window.innerHeight - element.getBoundingClientRect().bottom)
+        )
         .toBeGreaterThanOrEqual(0);
     const mobileToastGeometry = await commentToast.evaluate((element) => {
         const rect = element.getBoundingClientRect();
diff --git a/app/game-frontend/src/utils/legacyDateTime.ts b/app/game-frontend/src/utils/legacyDateTime.ts
index 3b89fd7e..38ea3b55 100644
--- a/app/game-frontend/src/utils/legacyDateTime.ts
+++ b/app/game-frontend/src/utils/legacyDateTime.ts
@@ -1,24 +1,6 @@
-const KOREA_TIME_OFFSET_MS = 9 * 60 * 60 * 1000;
+import { formatServerDateTime } from '@sammo-ts/common';
 
-const pad = (value: number): string => String(value).padStart(2, '0');
+export const formatSeoulDateTime = (value: string | Date): string => formatServerDateTime(value);
 
-export const formatSeoulDateTime = (value: string | Date): string => {
-    if (
-        typeof value === 'string' &&
-        !/(?:Z|[+-]\d{2}:?\d{2})$/i.test(value.trim())
-    ) {
-        return value.trim().replace('T', ' ').slice(0, 19);
-    }
-    const date = value instanceof Date ? value : new Date(value);
-    if (Number.isNaN(date.getTime())) {
-        return typeof value === 'string' ? value.slice(0, 19) : '';
-    }
-    const koreaTime = new Date(date.getTime() + KOREA_TIME_OFFSET_MS);
-    return `${koreaTime.getUTCFullYear()}-${pad(koreaTime.getUTCMonth() + 1)}-${pad(
-        koreaTime.getUTCDate()
-    )} ${pad(koreaTime.getUTCHours())}:${pad(koreaTime.getUTCMinutes())}:${pad(
-        koreaTime.getUTCSeconds()
-    )}`;
-};
-
-export const formatSeoulHourMinute = (value: string | Date): string => formatSeoulDateTime(value).slice(11, 16);
+export const formatSeoulHourMinute = (value: string | Date): string =>
+    formatServerDateTime(value, { format: 'hourMinute' });
diff --git a/app/game-frontend/src/views/AuctionView.vue b/app/game-frontend/src/views/AuctionView.vue
index dfb03396..fa3c4c58 100644
--- a/app/game-frontend/src/views/AuctionView.vue
+++ b/app/game-frontend/src/views/AuctionView.vue
@@ -1,4 +1,5 @@
 <script setup lang="ts">
+import { formatServerDateTime } from '@sammo-ts/common';
 import { computed, onMounted, reactive, ref, watch } from 'vue';
 import { useRoute } from 'vue-router';
 
@@ -41,24 +42,10 @@ const formatNumber = (value: number | null | undefined): string => (value ?? 0).
 const displayCode = (value: string | null | undefined): string =>
     !value || /^\d+$/u.test(value) ? '-' : value.replace(/^che_(?:event_)?/u, '');
 const cutDateTime = (value: string | null | undefined, showSecond = false): string => {
-    if (!value) {
-        return '-';
-    }
-    const date = new Date(value);
-    if (Number.isNaN(date.getTime())) {
-        return value.slice(5, showSecond ? 19 : 16);
-    }
-    const parts = new Intl.DateTimeFormat('ko-KR', {
-        month: '2-digit',
-        day: '2-digit',
-        hour: '2-digit',
-        minute: '2-digit',
-        ...(showSecond ? { second: '2-digit' } : {}),
-        hour12: false,
-    }).formatToParts(date);
-    const part = (type: Intl.DateTimeFormatPartTypes): string =>
-        parts.find((entry) => entry.type === type)?.value ?? '';
-    return `${part('month')}-${part('day')} ${part('hour')}:${part('minute')}${showSecond ? `:${part('second')}` : ''}`;
+    return formatServerDateTime(value, {
+        format: showSecond ? 'monthDayTimeSeconds' : 'monthDayTime',
+        fallback: '-',
+    });
 };
 
 const buyRice = computed(() =>
diff --git a/app/game-frontend/src/views/BattleCenterView.vue b/app/game-frontend/src/views/BattleCenterView.vue
index a71eb31d..fd697209 100644
--- a/app/game-frontend/src/views/BattleCenterView.vue
+++ b/app/game-frontend/src/views/BattleCenterView.vue
@@ -1,4 +1,5 @@
 <script setup lang="ts">
+import { formatServerDateTime } from '@sammo-ts/common';
 import { computed, onMounted, reactive, ref, watch } from 'vue';
 import { useRoute } from 'vue-router';
 import PanelCard from '../components/ui/PanelCard.vue';
@@ -126,9 +127,9 @@ const selectedGeneral = computed(() => {
 
 const formatGeneralLabel = (general: GeneralEntry): string => {
     const name = general.officerLevel > 4 ? `*${general.name}*` : general.name;
-    const time = general.turnTime ? general.turnTime.slice(-5) : '--:--';
+    const time = formatServerDateTime(general.turnTime, { format: 'hourMinute', fallback: '--:--' });
     if (orderBy.value === 'recentWar') {
-        return `${name} (${general.recentWar ? general.recentWar.slice(-5) : '--:--'})`;
+        return `${name} (${formatServerDateTime(general.recentWar, { format: 'hourMinute', fallback: '--:--' })})`;
     }
     if (orderBy.value === 'warnum') {
         return `${name} (${general.warnum}회)`;
@@ -156,7 +157,10 @@ const loadLogs = async (generalId: number) => {
         }
         for (const response of responses) {
             const formatted = response.logs.map((entry) => {
-                const eventTime = response.type === 'generalAction' ? ` ${entry.createdAt.slice(-8, -3)}` : '';
+                const eventTime =
+                    response.type === 'generalAction'
+                        ? ` ${formatServerDateTime(entry.createdAt, { format: 'hourMinute' })}`
+                        : '';
                 return {
                     id: entry.id,
                     html: formatLog(`${entry.text}${eventTime}`),
@@ -308,7 +312,12 @@ onMounted(() => {
                         <LegacyGeneralProgress :general="selectedGeneral" />
                     </div>
                     <div v-if="selectedGeneral" class="general-meta">
-                        <div>최근 턴: {{ selectedGeneral.turnTime ? selectedGeneral.turnTime.slice(-5) : '-' }}</div>
+                        <div>
+                            최근 턴:
+                            {{
+                                formatServerDateTime(selectedGeneral.turnTime, { format: 'hourMinute', fallback: '-' })
+                            }}
+                        </div>
                         <div>최근 전투: {{ selectedGeneral.recentWar || '-' }}</div>
                         <div>전투 횟수: {{ selectedGeneral.warnum }}</div>
                     </div>
diff --git a/app/game-frontend/src/views/BettingView.vue b/app/game-frontend/src/views/BettingView.vue
index 90242ce5..fd1f8f6e 100644
--- a/app/game-frontend/src/views/BettingView.vue
+++ b/app/game-frontend/src/views/BettingView.vue
@@ -1,4 +1,5 @@
 <script setup lang="ts">
+import { formatServerDateTime } from '@sammo-ts/common';
 import { computed, onMounted, ref } from 'vue';
 import TournamentBracket from '../components/tournament/TournamentBracket.vue';
 import { trpc } from '../utils/trpc';
@@ -68,7 +69,9 @@ const ratio = (id: number) => {
     const amount = totals?.[id] ?? 0;
     return amount ? (totalAmount.value / amount).toFixed(2) : '0';
 };
-const openingTime = computed(() => snapshot.value?.state?.nextAt?.slice(11, 16) ?? '--:--');
+const openingTime = computed(() =>
+    formatServerDateTime(snapshot.value?.state?.nextAt, { format: 'hourMinute', fallback: '--:--' })
+);
 const expected = (id: number) => {
     const myTotals = summary.value?.myTotals as Record<number, number> | undefined;
     const current = myTotals?.[id] ?? 0;
@@ -248,8 +251,7 @@ const placeBet = async (targetId: number) => {
                 <button class="close-button" type="button" @click="navigate">창 닫기</button>
             </RouterLink>
             <small>
-                삼국지 모의전투 HiDCHe / KOEI의 이미지를 사용, 응용하였습니다 / 제작 :
-                HideD(hided62@gmail.com) / Credit
+                삼국지 모의전투 HiDCHe / KOEI의 이미지를 사용, 응용하였습니다 / 제작 : HideD(hided62@gmail.com) / Credit
             </small>
         </footer>
     </main>
diff --git a/app/game-frontend/src/views/BoardView.vue b/app/game-frontend/src/views/BoardView.vue
index 1622bb91..a17d5c1b 100644
--- a/app/game-frontend/src/views/BoardView.vue
+++ b/app/game-frontend/src/views/BoardView.vue
@@ -1,4 +1,5 @@
 <script setup lang="ts">
+import { formatServerDateTime } from '@sammo-ts/common';
 import { computed, nextTick, onMounted, reactive, ref, watch } from 'vue';
 import { useRoute, useRouter } from 'vue-router';
 
@@ -40,7 +41,7 @@ const resizeTextArea = (element: HTMLTextAreaElement | null) => {
     element.style.height = `${Math.max(element.scrollHeight, 42)}px`;
 };
 
-const formatDate = (value: string): string => value.slice(5, 16).replace('T', ' ');
+const formatDate = (value: string): string => formatServerDateTime(value, { format: 'monthDayTime' });
 
 const iconPath = (article: BoardArticle): string =>
     resolveGeneralIconUrl({
@@ -160,7 +161,14 @@ onMounted(() => {
                 </div>
                 <div class="article-submit-row">
                     <div></div>
-                    <button id="submitArticle" class="legacy-button legacy-button--secondary" type="button" @click="submitArticle">등록</button>
+                    <button
+                        id="submitArticle"
+                        class="legacy-button legacy-button--secondary"
+                        type="button"
+                        @click="submitArticle"
+                    >
+                        등록
+                    </button>
                 </div>
             </section>
 
@@ -244,7 +252,9 @@ onMounted(() => {
     padding: 8px;
     color: #000;
     background: #fff;
-    font: 16px/normal 'Times New Roman', serif;
+    font:
+        16px/normal 'Times New Roman',
+        serif;
 }
 
 .legacy-board-page {
diff --git a/app/game-frontend/src/views/DynastyDetailView.vue b/app/game-frontend/src/views/DynastyDetailView.vue
index c9c28664..15107fd4 100644
--- a/app/game-frontend/src/views/DynastyDetailView.vue
+++ b/app/game-frontend/src/views/DynastyDetailView.vue
@@ -1,4 +1,5 @@
 <script setup lang="ts">
+import { formatServerDateTime } from '@sammo-ts/common';
 import { computed, onMounted, ref, watch } from 'vue';
 import { useRoute, useRouter } from 'vue-router';
 
@@ -47,12 +48,7 @@ const loadDetail = async (): Promise<void> => {
     }
 };
 
-const formatArchiveDate = (value: string): string =>
-    new Intl.DateTimeFormat('sv-SE', {
-        dateStyle: 'short',
-        timeStyle: 'medium',
-        timeZone: 'UTC',
-    }).format(new Date(value));
+const formatArchiveDate = (value: string): string => formatServerDateTime(value);
 
 watch(emperorId, loadDetail);
 onMounted(loadDetail);
@@ -67,7 +63,9 @@ onMounted(loadDetail);
                         역 대 왕 조<br />
                         <button class="native-button" type="button" @click="closePage">창 닫기</button>
                         <span class="all-link">
-                            <RouterLink to="/dynasty"><button class="native-button" type="button">전체보기</button></RouterLink>
+                            <RouterLink to="/dynasty"
+                                ><button class="native-button" type="button">전체보기</button></RouterLink
+                            >
                         </span>
                     </td>
                 </tr>
@@ -202,7 +200,11 @@ onMounted(loadDetail);
                         <td colspan="5">
                             <!-- 레거시 색상 tag를 동일한 span 구조로 변환한다. -->
                             <!-- eslint-disable-next-line vue/no-v-html -->
-                            <div v-for="(entry, index) in data.emperor.history" :key="index" v-html="formatLog(entry)" />
+                            <div
+                                v-for="(entry, index) in data.emperor.history"
+                                :key="index"
+                                v-html="formatLog(entry)"
+                            />
                         </td>
                     </tr>
                 </tbody>
@@ -283,14 +285,10 @@ onMounted(loadDetail);
         <table class="legacy-table legacy-bg0 footer-table">
             <tbody>
                 <tr>
-                    <td>
-                        <button class="native-button" type="button" @click="closePage">창 닫기</button><br />
-                    </td>
+                    <td><button class="native-button" type="button" @click="closePage">창 닫기</button><br /></td>
                 </tr>
                 <tr>
-                    <td class="banner">
-                        삼국지 모의전투 HiDCHe / KOEI의 이미지를 사용, 응용하였습니다 / 제작 : HideD
-                    </td>
+                    <td class="banner">삼국지 모의전투 HiDCHe / KOEI의 이미지를 사용, 응용하였습니다 / 제작 : HideD</td>
                 </tr>
             </tbody>
         </table>
diff --git a/app/game-frontend/src/views/InheritView.vue b/app/game-frontend/src/views/InheritView.vue
index 40f00306..e22ae871 100644
--- a/app/game-frontend/src/views/InheritView.vue
+++ b/app/game-frontend/src/views/InheritView.vue
@@ -1,4 +1,5 @@
 <script setup lang="ts">
+import { formatServerDateTime } from '@sammo-ts/common';
 import { computed, onMounted, reactive, ref } from 'vue';
 import { trpc } from '../utils/trpc';
 
@@ -169,11 +170,7 @@ const turnTimeLabel = computed(() => {
     if (!turnTimeResult.value) {
         return null;
     }
-    const parsed = new Date(turnTimeResult.value);
-    if (Number.isNaN(parsed.getTime())) {
-        return turnTimeResult.value;
-    }
-    return parsed.toLocaleString();
+    return formatServerDateTime(turnTimeResult.value);
 });
 
 const isUnited = computed(() => status.value?.isUnited ?? false);
@@ -735,7 +732,7 @@ onMounted(() => {
                 <div v-if="logLoading && logs.length === 0" class="log-empty">불러오는 중...</div>
                 <div v-else-if="logs.length === 0" class="log-empty">기록이 없습니다.</div>
                 <div v-for="entry in logs" v-else :key="entry.id" class="log-row">
-                    <small>[{{ new Date(entry.createdAt).toLocaleString('ko-KR') }}]</small>
+                    <small>[{{ formatServerDateTime(entry.createdAt) }}]</small>
                     <span>{{ entry.text }}</span>
                 </div>
                 <button
diff --git a/app/game-frontend/src/views/MainView.vue b/app/game-frontend/src/views/MainView.vue
index 886e1cec..2f7508cb 100644
--- a/app/game-frontend/src/views/MainView.vue
+++ b/app/game-frontend/src/views/MainView.vue
@@ -1,4 +1,5 @@
 <script setup lang="ts">
+import { formatServerDateTime } from '@sammo-ts/common';
 import { computed, onMounted, onUnmounted, ref, watch } from 'vue';
 import { storeToRefs } from 'pinia';
 import { useMediaQuery } from '@vueuse/core';
@@ -68,14 +69,8 @@ const nationColor = computed(() => nation.value?.color ?? '#000000');
 const voteActive = computed(() => Boolean(frontStatus.value?.latestVote));
 const formatRecord = (entry: { text: string; createdAt?: string | Date }, appendTime = false): string => {
     if (!appendTime || /\d{2}:\d{2}\s*$/u.test(entry.text)) return formatLog(entry.text);
-    const parsed = entry.createdAt ? new Date(entry.createdAt) : null;
-    if (!parsed || Number.isNaN(parsed.getTime())) return formatLog(entry.text);
-    const time = new Intl.DateTimeFormat('ko-KR', {
-        timeZone: 'Asia/Seoul',
-        hour: '2-digit',
-        minute: '2-digit',
-        hour12: false,
-    }).format(parsed);
+    const time = formatServerDateTime(entry.createdAt, { format: 'hourMinute', fallback: '' });
+    if (!time) return formatLog(entry.text);
     return formatLog(`${entry.text} ${time}`);
 };
 
diff --git a/app/game-frontend/src/views/MyPageView.vue b/app/game-frontend/src/views/MyPageView.vue
index 7b9859e3..45cbd49b 100644
--- a/app/game-frontend/src/views/MyPageView.vue
+++ b/app/game-frontend/src/views/MyPageView.vue
@@ -2,7 +2,7 @@
 import { computed, onMounted, reactive, ref, watch } from 'vue';
 import { trpc } from '../utils/trpc';
 import { formatLog } from '../utils/formatLog';
-import { formatSeoulDateTime } from '../utils/legacyDateTime';
+import { formatSeoulDateTime, formatSeoulHourMinute } from '../utils/legacyDateTime';
 import { isDefenceTrainPenaltyWaivedByScenarioEffect } from '@sammo-ts/logic';
 import { useSessionStore } from '../stores/session';
 import { resolveGeneralIconUrl, useDefaultGeneralIcon } from '../utils/generalIcon';
@@ -430,7 +430,10 @@ onMounted(() => {
                         </div>
                         <div>
                             <dt>나이/다음턴</dt>
-                            <dd>{{ data.general.age ?? '-' }}세 / {{ data.general.turnTime?.slice(11, 16) ?? '-' }}</dd>
+                            <dd>
+                                {{ data.general.age ?? '-' }}세 /
+                                {{ data.general.turnTime ? formatSeoulHourMinute(data.general.turnTime) : '-' }}
+                            </dd>
                         </div>
                     </dl>
                 </div>
diff --git a/app/game-frontend/src/views/NationSecretView.vue b/app/game-frontend/src/views/NationSecretView.vue
index ca047476..1c5d1f9d 100644
--- a/app/game-frontend/src/views/NationSecretView.vue
+++ b/app/game-frontend/src/views/NationSecretView.vue
@@ -1,4 +1,5 @@
 <script setup lang="ts">
+import { formatServerDateTime } from '@sammo-ts/common';
 import { computed, onMounted, ref } from 'vue';
 import { trpc } from '../utils/trpc';
 type Result = Awaited<ReturnType<typeof trpc.nation.getSecretGeneralList.query>>;
@@ -147,7 +148,7 @@ onMounted(load);
                             >
                         </td>
                         <td>{{ general.killTurn }}</td>
-                        <td>{{ general.turnTime.slice(14, 19) }}</td>
+                        <td>{{ formatServerDateTime(general.turnTime, { format: 'minuteSecond' }) }}</td>
                     </tr>
                 </tbody>
             </table>
@@ -159,8 +160,8 @@ onMounted(load);
                 </tr>
                 <tr>
                     <td class="legacy-banner">
-                        삼국지 모의전투 HiDCHe / KOEI의 이미지를 사용, 응용하였습니다 / 제작 :
-                        HideD(hided62@gmail.com) /
+                        삼국지 모의전투 HiDCHe / KOEI의 이미지를 사용, 응용하였습니다 / 제작 : HideD(hided62@gmail.com)
+                        /
                         <a href="https://github.com/hided/SamK" target="_blank" rel="noopener noreferrer">Credit</a>
                     </td>
                 </tr>
diff --git a/app/game-frontend/src/views/SurveyView.vue b/app/game-frontend/src/views/SurveyView.vue
index e5281642..88761564 100644
--- a/app/game-frontend/src/views/SurveyView.vue
+++ b/app/game-frontend/src/views/SurveyView.vue
@@ -1,4 +1,5 @@
 <script setup lang="ts">
+import { formatServerDateTime } from '@sammo-ts/common';
 import { computed, onMounted, ref } from 'vue';
 import { useRouter } from 'vue-router';
 
@@ -67,16 +68,9 @@ const newVoteOptions = computed(() => newVoteOptionsText.value.split('\n').filte
 
 const percentage = (count: number, total: number): string => ((count / Math.max(1, total)) * 100).toFixed(1);
 
-const formatStartDate = (value: string): string => value.slice(0, 10);
+const formatStartDate = (value: string): string => formatServerDateTime(value, { format: 'date' });
 
-const formatCommentDate = (value: string): string => {
-    const date = new Date(value);
-    if (Number.isNaN(date.getTime())) {
-        return value;
-    }
-    const pad = (part: number) => String(part).padStart(2, '0');
-    return `${pad(date.getMonth() + 1)}-${pad(date.getDate())} ${pad(date.getHours())}:${pad(date.getMinutes())}`;
-};
+const formatCommentDate = (value: string): string => formatServerDateTime(value, { format: 'monthDayTime' });
 
 const voteColor = (index: number): string =>
     ['#ff0000', '#ffa500', '#ffff00', '#008000', '#0000ff', '#000080', '#800080'][index % 7]!;
diff --git a/app/game-frontend/src/views/TournamentView.vue b/app/game-frontend/src/views/TournamentView.vue
index 9175059c..8336070f 100644
--- a/app/game-frontend/src/views/TournamentView.vue
+++ b/app/game-frontend/src/views/TournamentView.vue
@@ -1,4 +1,5 @@
 <script setup lang="ts">
+import { formatServerDateTime } from '@sammo-ts/common';
 import { computed, onMounted, ref } from 'vue';
 import TournamentBracket from '../components/tournament/TournamentBracket.vue';
 import { trpc } from '../utils/trpc';
@@ -49,7 +50,9 @@ const matchesAt = (stage: number) =>
         .sort((a, b) => a.roundIndex - b.roundIndex);
 const nameOf = (id?: number) => (id ? (participantsById.value.get(id)?.name ?? `#${id}`) : '-');
 const totalBet = computed(() => betting.value?.totalAmount ?? 0);
-const openingTime = computed(() => snapshot.value?.state?.nextAt?.slice(11, 16) ?? '--:--');
+const openingTime = computed(() =>
+    formatServerDateTime(snapshot.value?.state?.nextAt, { format: 'hourMinute', fallback: '--:--' })
+);
 const betTotals = computed(() => betting.value?.totals as Record<number, number> | undefined);
 const isParticipant = computed(() =>
     (snapshot.value?.participants ?? []).some((participant) => participant.id === myGeneralId.value)
@@ -274,8 +277,7 @@ const start = async () => {
                 <button class="close-button" type="button" @click="navigate">창 닫기</button>
             </RouterLink>
             <small>
-                삼국지 모의전투 HiDCHe / KOEI의 이미지를 사용, 응용하였습니다 / 제작 :
-                HideD(hided62@gmail.com) / Credit
+                삼국지 모의전투 HiDCHe / KOEI의 이미지를 사용, 응용하였습니다 / 제작 : HideD(hided62@gmail.com) / Credit
             </small>
         </footer>
 
diff --git a/app/game-frontend/src/views/TrafficView.vue b/app/game-frontend/src/views/TrafficView.vue
index b4830a25..efdbb748 100644
--- a/app/game-frontend/src/views/TrafficView.vue
+++ b/app/game-frontend/src/views/TrafficView.vue
@@ -1,4 +1,5 @@
 <script setup lang="ts">
+import { formatServerDateTime } from '@sammo-ts/common';
 import { computed, onMounted, ref } from 'vue';
 import { useRouter } from 'vue-router';
 
@@ -50,10 +51,7 @@ const onlineRows = computed(() =>
     }))
 );
 
-const timeLabel = (value: string): string => {
-    const timePart = value.includes('T') ? value.split('T')[1] : value.slice(11);
-    return (timePart ?? '').slice(0, 5);
-};
+const timeLabel = (value: string): string => formatServerDateTime(value, { format: 'hourMinute' });
 
 const trafficColor = (percentage: number): string => {
     const channel = (value: number): string =>
@@ -204,8 +202,8 @@ onMounted(() => {
                 </tr>
                 <tr>
                     <td class="banner">
-                        삼국지 모의전투 HiDCHe / KOEI의 이미지를 사용, 응용하였습니다 / 제작 :
-                        HideD(hided62@gmail.com) /
+                        삼국지 모의전투 HiDCHe / KOEI의 이미지를 사용, 응용하였습니다 / 제작 : HideD(hided62@gmail.com)
+                        /
                         <a href="https://sam.hided.net/wiki/hidche/credit" target="_blank" rel="noreferrer">Credit</a>
                     </td>
                 </tr>
diff --git a/app/game-frontend/src/views/TroopView.vue b/app/game-frontend/src/views/TroopView.vue
index 47785e52..587349f8 100644
--- a/app/game-frontend/src/views/TroopView.vue
+++ b/app/game-frontend/src/views/TroopView.vue
@@ -1,4 +1,5 @@
 <script setup lang="ts">
+import { formatServerDateTime } from '@sammo-ts/common';
 import { computed, onMounted, ref } from 'vue';
 import { useRouter } from 'vue-router';
 
@@ -166,10 +167,7 @@ const hideMemberPopup = () => {
 const iconPath = (troop: Troop): string => resolveGeneralIconUrl(troop.leader ?? {});
 
 const formatTurn = (turnTime: string | null): string => {
-    if (!turnTime) {
-        return '--:--';
-    }
-    return turnTime.slice(14, 19);
+    return formatServerDateTime(turnTime, { format: 'minuteSecond', fallback: '--:--' });
 };
 
 onMounted(() => {
diff --git a/app/gateway-frontend/e2e/server-operations.spec.ts b/app/gateway-frontend/e2e/server-operations.spec.ts
index 3da117a6..d369b41e 100644
--- a/app/gateway-frontend/e2e/server-operations.spec.ts
+++ b/app/gateway-frontend/e2e/server-operations.spec.ts
@@ -490,6 +490,7 @@ test('separates branch and commit semantics and submits a reset from the dedicat
     await page.getByTestId('source-ref').fill('0123456789abcdef0123456789abcdef01234567');
     await page.getByTestId('load-scenarios').click();
     await page.getByTestId('scenario-select').selectOption('5');
+    await page.getByLabel('작업 예약 (서버 시간 UTC+9)').fill('2026-08-13T09:30');
     await page.getByTestId('request-reset').hover();
     await page.getByTestId('request-reset').click();
 
@@ -544,6 +545,7 @@ test('separates branch and commit semantics and submits a reset from the dedicat
     expect(JSON.stringify(resetRequest?.body)).toContain('"sourceMode":"COMMIT"');
     expect(JSON.stringify(resetRequest?.body)).toContain('0123456789abcdef0123456789abcdef01234567');
     expect(JSON.stringify(resetRequest?.body)).toContain('"scenarioId":5');
+    expect(JSON.stringify(resetRequest?.body)).toContain('"scheduledAt":"2026-08-13T00:30:00.000Z"');
     await page.screenshot({ path: testInfo.outputPath('reset-operation-log-desktop.png'), fullPage: true });
 
     await page.setViewportSize({ width: 390, height: 844 });
@@ -588,9 +590,9 @@ test('separates branch and commit semantics and submits a reset from the dedicat
         mobileOperationTableGeometry.scrollerWidth
     );
     expect(mobileOperationTableGeometry.scrollerX).toBeGreaterThanOrEqual(0);
-    expect(
-        mobileOperationTableGeometry.scrollerX + mobileOperationTableGeometry.scrollerWidth
-    ).toBeLessThanOrEqual(mobileOperationTableGeometry.viewportWidth);
+    expect(mobileOperationTableGeometry.scrollerX + mobileOperationTableGeometry.scrollerWidth).toBeLessThanOrEqual(
+        mobileOperationTableGeometry.viewportWidth
+    );
     expect(mobileOperationTableGeometry.documentScrollWidth).toBeLessThanOrEqual(
         mobileOperationTableGeometry.viewportWidth
     );
diff --git a/app/gateway-frontend/src/views/AccountView.vue b/app/gateway-frontend/src/views/AccountView.vue
index b6519969..bf39993b 100644
--- a/app/gateway-frontend/src/views/AccountView.vue
+++ b/app/gateway-frontend/src/views/AccountView.vue
@@ -1,4 +1,5 @@
 <script setup lang="ts">
+import { formatServerDateTime } from '@sammo-ts/common';
 import { computed, nextTick, onBeforeUnmount, onMounted, ref, watch } from 'vue';
 import { useRouter } from 'vue-router';
 
@@ -130,7 +131,7 @@ const scheduleDeletion = async (): Promise<void> => {
             currentCredential,
         });
         window.localStorage.removeItem('sammo-session-token');
-        successMessage.value = `${new Date(result.deleteAfter).toLocaleDateString('ko-KR')}까지 정보가 보존됩니다.`;
+        successMessage.value = `${formatServerDateTime(result.deleteAfter, { format: 'date' })}까지 정보가 보존됩니다.`;
         await router.replace('/');
     });
 };
@@ -411,7 +412,7 @@ onBeforeUnmount(() => {
                     </tr>
                     <tr>
                         <th class="legacy-bg1">가입일시</th>
-                        <td colspan="2">{{ new Date(account.createdAt).toLocaleString('ko-KR') }}</td>
+                        <td colspan="2">{{ formatServerDateTime(account.createdAt) }}</td>
                         <td colspan="3">
                             개인정보 3자 제공 동의 : {{ account.thirdPartyUse ? '○' : '×' }}
                             <button
diff --git a/app/gateway-frontend/src/views/AdminView.vue b/app/gateway-frontend/src/views/AdminView.vue
index 360b7212..2f1da60c 100644
--- a/app/gateway-frontend/src/views/AdminView.vue
+++ b/app/gateway-frontend/src/views/AdminView.vue
@@ -1,4 +1,5 @@
 <script setup lang="ts">
+import { formatServerDateTime, serverDateTimeInputToIso, toServerDateTimeInputValue } from '@sammo-ts/common';
 import { computed, onMounted, ref, watch } from 'vue';
 import ServerProfileTabs from '../components/ServerProfileTabs.vue';
 import AdminConsoleLayout from '../layouts/AdminConsoleLayout.vue';
@@ -435,8 +436,7 @@ const runtimeActionStatusClass = (status: AdminProfile['runtimeActions'][number]
 const isRuntimeActionTerminal = (status: AdminProfile['runtimeActions'][number]['status']): boolean =>
     status === 'APPLIED' || status === 'FAILED' || status === 'IGNORED';
 
-const formatRuntimeActionTime = (value: string | null): string =>
-    value ? new Date(value).toLocaleString('ko-KR') : '';
+const formatRuntimeActionTime = (value: string | null): string => formatServerDateTime(value);
 
 const userLookupMode = ref<'username' | 'id' | 'email'>('username');
 const userLookupValue = ref('');
@@ -630,14 +630,7 @@ const ensureProfileBuffers = (profile: AdminProfile) => {
     }
 };
 
-const toLocalInputValue = (value: string): string => {
-    const date = new Date(value);
-    if (Number.isNaN(date.getTime())) return '';
-    const pad = (part: number): string => String(part).padStart(2, '0');
-    return `${date.getFullYear()}-${pad(date.getMonth() + 1)}-${pad(date.getDate())}T${pad(date.getHours())}:${pad(
-        date.getMinutes()
-    )}`;
-};
+const toLocalInputValue = (value: string): string => toServerDateTimeInputValue(value);
 
 const loadProfiles = async () => {
     profilesLoading.value = true;
@@ -795,7 +788,7 @@ const requestProfileAction = async (profileName: string, action: AdminAction) =>
     const durationValue = durationMinutes && validDuration(profileName) ? durationMinutes : undefined;
     const scheduledAt =
         action === 'RESET_SCHEDULED' && actionState?.scheduledAt
-            ? new Date(actionState.scheduledAt).toISOString()
+            ? serverDateTimeInputToIso(actionState.scheduledAt)
             : undefined;
     const reason = actionState?.reason.trim() || undefined;
     let runtimeActionId: string | undefined;
@@ -952,7 +945,7 @@ const updateKakaoGrace = async (clear = false) => {
     try {
         const result = await adminClient.users.updateKakaoGrace.mutate({
             userId: userResult.value.id,
-            until: clear || !kakaoGraceUntil.value ? null : new Date(kakaoGraceUntil.value).toISOString(),
+            until: clear || !kakaoGraceUntil.value ? null : (serverDateTimeInputToIso(kakaoGraceUntil.value) ?? null),
             reason,
         });
         userResult.value = {
@@ -983,7 +976,9 @@ const grantSpecialAccess = async () => {
                 .map((profile) => profile.trim())
                 .filter(Boolean),
             allowsGeneralCreation: specialAccessAllowsGeneralCreation.value,
-            expiresAt: specialAccessExpiresAt.value ? new Date(specialAccessExpiresAt.value).toISOString() : null,
+            expiresAt: specialAccessExpiresAt.value
+                ? (serverDateTimeInputToIso(specialAccessExpiresAt.value) ?? null)
+                : null,
             reason,
         });
         const policy = await adminClient.users.getKakaoGracePolicies.query({ userId: userResult.value.id });
@@ -1071,7 +1066,7 @@ const applyBan = async () => {
     }
     const reason = requireUserActionReason();
     if (!reason) return;
-    const until = banUntil.value ? new Date(banUntil.value).toISOString() : null;
+    const until = banUntil.value ? (serverDateTimeInputToIso(banUntil.value) ?? null) : null;
     const patch = {
         bannedUntil: until,
         notes: banReason.value.trim() || undefined,
@@ -1150,7 +1145,7 @@ const applyRestriction = async () => {
         .filter(Boolean);
     const restriction = {
         blockedFeatures: features.length ? features : undefined,
-        until: restrictionUntil.value ? new Date(restrictionUntil.value).toISOString() : undefined,
+        until: restrictionUntil.value ? serverDateTimeInputToIso(restrictionUntil.value) : undefined,
         reason: restrictionReason.value.trim() || undefined,
         notes: restrictionNotes.value.trim() || undefined,
     };
@@ -1213,7 +1208,7 @@ const scheduleDeleteUser = async () => {
             reason,
         });
         userResult.value = { ...userResult.value, deleteAfter: result.deleteAfter };
-        forceDeleteStatus.value = `탈퇴 예약 완료: ${new Date(result.deleteAfter).toLocaleString('ko-KR')}`;
+        forceDeleteStatus.value = `탈퇴 예약 완료: ${formatServerDateTime(result.deleteAfter)}`;
         await Promise.all([refreshUserHistory(), loadUserDirectory()]);
     } catch (error) {
         forceDeleteStatus.value = '탈퇴 예약 실패';
@@ -1357,7 +1352,7 @@ onMounted(() => {
                                     <span class="block truncate">{{ user.email || '이메일 없음' }}</span>
                                     <span
                                         >{{ user.oauthType }} ·
-                                        {{ new Date(user.createdAt).toLocaleDateString('ko-KR') }}</span
+                                        {{ formatServerDateTime(user.createdAt, { format: 'date' }) }}</span
                                     >
                                 </span>
                                 <span class="flex flex-wrap gap-1 md:justify-end">
@@ -1436,15 +1431,17 @@ onMounted(() => {
                             </div>
                             <div class="text-xs text-zinc-500">
                                 Kakao 인증: {{ userResult.kakaoVerifiedAt ? '완료' : '미완료' }} · 유예 시작:
-                                {{ new Date(userResult.kakaoGraceStartedAt).toLocaleString('ko-KR') }}
+                                {{ formatServerDateTime(userResult.kakaoGraceStartedAt) }}
                             </div>
                             <div v-if="userResult.kakaoGraceUntil" class="text-xs text-amber-300">
-                                관리자 유예: {{ new Date(userResult.kakaoGraceUntil).toLocaleString('ko-KR') }}까지
+                                관리자 유예: {{ formatServerDateTime(userResult.kakaoGraceUntil) }}까지
                             </div>
                             <div v-if="userResult.deleteAfter" class="text-xs text-red-300">
-                                탈퇴 예약: {{ new Date(userResult.deleteAfter).toLocaleString('ko-KR') }}
+                                탈퇴 예약: {{ formatServerDateTime(userResult.deleteAfter) }}
+                            </div>
+                            <div class="text-xs text-zinc-500">
+                                가입일: {{ formatServerDateTime(userResult.createdAt) }}
                             </div>
-                            <div class="text-xs text-zinc-500">가입일: {{ userResult.createdAt }}</div>
                             <div class="text-xs text-zinc-400 mt-2">제재 상태</div>
                             <pre class="text-[11px] text-zinc-400 bg-black/50 p-2 rounded whitespace-pre-wrap"
                                 >{{ JSON.stringify(userResult.sanctions, null, 2) }}
@@ -1632,7 +1629,8 @@ onMounted(() => {
                         <h4 class="text-base font-semibold">Kakao 없는 특수 계정 접근</h4>
                         <div class="text-xs text-zinc-400">
                             운영자 role은 자동으로 모든 서버에 접근합니다. 테스트·복구·기타 계정은 아래에서 서버 범위와
-                            만료를 명시해 부여합니다. 복구 자격은 만료가 필수이며 최대 90일입니다.
+                            만료를 명시해 부여합니다. 복구 자격은 만료가 필수이며 최대 90일입니다. 시각 입력은 서버 시간
+                            UTC+9 기준입니다.
                         </div>
                         <div class="grid grid-cols-1 md:grid-cols-2 gap-2">
                             <select
@@ -1696,11 +1694,11 @@ onMounted(() => {
                                 </div>
                                 <div>
                                     장수 생성 {{ grant.allowsGeneralCreation ? '허용' : '차단' }} · 만료
-                                    {{ grant.expiresAt ? new Date(grant.expiresAt).toLocaleString('ko-KR') : '없음' }}
+                                    {{ formatServerDateTime(grant.expiresAt, { fallback: '없음' }) }}
                                 </div>
                                 <div class="text-zinc-500">부여 사유: {{ grant.reason }}</div>
                                 <div v-if="grant.revokedAt" class="text-red-300">
-                                    해제됨: {{ new Date(grant.revokedAt).toLocaleString('ko-KR') }} ·
+                                    해제됨: {{ formatServerDateTime(grant.revokedAt) }} ·
                                     {{ grant.revokedReason }}
                                 </div>
                             </div>
@@ -1713,7 +1711,8 @@ onMounted(() => {
                     >
                         <h4 class="text-base font-semibold">Kakao 인증 유예</h4>
                         <div class="text-xs text-zinc-500">
-                            기본·서버별 유예가 끝난 사용자를 예외적으로 더 허용할 때 사용합니다.
+                            기본·서버별 유예가 끝난 사용자를 예외적으로 더 허용할 때 사용합니다. 시각 입력은 서버 시간
+                            UTC+9 기준입니다.
                         </div>
                         <div class="flex flex-col md:flex-row gap-2">
                             <input
@@ -1762,11 +1761,7 @@ onMounted(() => {
                                         <td class="text-center">{{ policy.accessGraceDays }}일</td>
                                         <td class="text-center">{{ policy.specialAccess?.kind ?? '-' }}</td>
                                         <td class="text-center">
-                                            {{
-                                                policy.graceEndsAt
-                                                    ? new Date(policy.graceEndsAt).toLocaleString('ko-KR')
-                                                    : '-'
-                                            }}
+                                            {{ policy.graceEndsAt ? formatServerDateTime(policy.graceEndsAt) : '-' }}
                                         </td>
                                     </tr>
                                 </tbody>
@@ -1778,7 +1773,7 @@ onMounted(() => {
                         v-if="userWorkspaceSection === 'restrictions'"
                         class="bg-zinc-900 border border-zinc-800 rounded-lg p-5 space-y-4"
                     >
-                        <h4 class="text-base font-semibold">유저 차단</h4>
+                        <h4 class="text-base font-semibold">유저 차단 (서버 시간 UTC+9)</h4>
                         <div class="flex flex-col gap-2">
                             <input
                                 v-model="banUntil"
@@ -1817,7 +1812,7 @@ onMounted(() => {
                         v-if="userWorkspaceSection === 'restrictions'"
                         class="bg-zinc-900 border border-zinc-800 rounded-lg p-5 space-y-4"
                     >
-                        <h4 class="text-base font-semibold">서버별 기능 제재</h4>
+                        <h4 class="text-base font-semibold">서버별 기능 제재 (서버 시간 UTC+9)</h4>
                         <div class="grid gap-2">
                             <input
                                 v-model="restrictionProfile"
@@ -1936,9 +1931,7 @@ onMounted(() => {
                                     >
                                         {{ event.outcome }} · {{ event.action }}
                                     </span>
-                                    <span class="text-zinc-500">{{
-                                        new Date(event.createdAt).toLocaleString('ko-KR')
-                                    }}</span>
+                                    <span class="text-zinc-500">{{ formatServerDateTime(event.createdAt) }}</span>
                                 </div>
                                 <div class="text-zinc-400">
                                     {{ event.actorUsername }} · {{ event.reason ?? '사유 없음' }}
@@ -1988,9 +1981,7 @@ onMounted(() => {
                                     >
                                         {{ event.outcome }} · {{ event.action }}
                                     </span>
-                                    <span class="text-zinc-500">{{
-                                        new Date(event.createdAt).toLocaleString('ko-KR')
-                                    }}</span>
+                                    <span class="text-zinc-500">{{ formatServerDateTime(event.createdAt) }}</span>
                                 </div>
                                 <div class="text-zinc-400">
                                     {{ event.actorUsername }} · {{ event.targetType ?? '-' }}
diff --git a/app/gateway-frontend/src/views/LobbyView.vue b/app/gateway-frontend/src/views/LobbyView.vue
index f4cd8ee3..d6b93ee8 100644
--- a/app/gateway-frontend/src/views/LobbyView.vue
+++ b/app/gateway-frontend/src/views/LobbyView.vue
@@ -1,4 +1,5 @@
 <script setup lang="ts">
+import { formatServerDateTime } from '@sammo-ts/common';
 import { computed, ref, onMounted, watch } from 'vue';
 import { useRouter } from 'vue-router';
 import type { inferRouterOutputs } from '@trpc/server';
@@ -95,8 +96,7 @@ const handleMapTabKeydown = (event: KeyboardEvent, profileName: string): void =>
     tabButtons?.[nextIndex]?.focus();
 };
 
-const formatGraceEndsAt = (value: string | null | undefined): string =>
-    value ? new Date(value).toLocaleString('ko-KR') : '';
+const formatGraceEndsAt = (value: string | null | undefined): string => formatServerDateTime(value);
 const serverSeasonStatus = (info: LobbyInfo) => resolveServerSeasonStatus(info);
 const encodeLegacyIconPath = (value: string): string =>
     value
diff --git a/app/gateway-frontend/src/views/ServerOperationsView.vue b/app/gateway-frontend/src/views/ServerOperationsView.vue
index 1894aa0b..7031f4d3 100644
--- a/app/gateway-frontend/src/views/ServerOperationsView.vue
+++ b/app/gateway-frontend/src/views/ServerOperationsView.vue
@@ -1,4 +1,5 @@
 <script setup lang="ts">
+import { formatServerDateTime, serverDateTimeInputToIso } from '@sammo-ts/common';
 import { computed, nextTick, onBeforeUnmount, onMounted, reactive, ref, watch } from 'vue';
 
 import ServerProfileTabs from '../components/ServerProfileTabs.vue';
@@ -213,21 +214,11 @@ const sourceHelp = computed(() =>
 );
 
 const toIso = (value: string): string | undefined => {
-    if (!value) {
-        return undefined;
-    }
-    const parsed = new Date(value);
-    return Number.isNaN(parsed.getTime()) ? undefined : parsed.toISOString();
+    return serverDateTimeInputToIso(value);
 };
 
-const formatTime = (value?: string): string => (value ? new Date(value).toLocaleString('ko-KR') : '-');
-const formatLogTime = (value: string): string =>
-    new Date(value).toLocaleTimeString('ko-KR', {
-        hour12: false,
-        hour: '2-digit',
-        minute: '2-digit',
-        second: '2-digit',
-    });
+const formatTime = (value?: string): string => formatServerDateTime(value, { fallback: '-' });
+const formatLogTime = (value: string): string => formatServerDateTime(value, { format: 'timeSeconds' });
 const shortSha = (value?: string): string => (value ? value.slice(0, 12) : '-');
 
 const clearStatus = () => {
@@ -959,7 +950,7 @@ onBeforeUnmount(() => {
 
                     <div v-if="mode === 'scenario'" class="grid gap-4 md:grid-cols-3">
                         <label class="text-xs text-zinc-400"
-                            >작업 예약
+                            >작업 예약 (서버 시간 UTC+9)
                             <input
                                 v-model="form.scheduledAt"
                                 type="datetime-local"
@@ -967,7 +958,7 @@ onBeforeUnmount(() => {
                             />
                         </label>
                         <label class="text-xs text-zinc-400"
-                            >가오픈
+                            >가오픈 (서버 시간 UTC+9)
                             <input
                                 v-model="form.preopenAt"
                                 type="datetime-local"
@@ -975,7 +966,7 @@ onBeforeUnmount(() => {
                             />
                         </label>
                         <label class="text-xs text-zinc-400"
-                            >정식 오픈
+                            >정식 오픈 (서버 시간 UTC+9)
                             <input
                                 v-model="form.openAt"
                                 type="datetime-local"
@@ -1302,10 +1293,7 @@ onBeforeUnmount(() => {
                     <span class="text-xs text-zinc-500">3초마다 상태 갱신</span>
                 </div>
                 <div class="overflow-x-auto">
-                    <table
-                        class="w-full min-w-[1300px] table-fixed text-left text-sm"
-                        data-testid="operations-table"
-                    >
+                    <table class="w-full min-w-[1300px] table-fixed text-left text-sm" data-testid="operations-table">
                         <colgroup>
                             <col style="width: 160px" />
                             <col style="width: 264px" />
diff --git a/packages/common/src/index.ts b/packages/common/src/index.ts
index dd0afbad..f5aa6489 100644
--- a/packages/common/src/index.ts
+++ b/packages/common/src/index.ts
@@ -1,6 +1,7 @@
 export * from './rng.js';
 export * from './time/Clock.js';
 export * from './time/GameClock.js';
+export * from './time/ServerDateTime.js';
 export * from './util/BytesLike.js';
 export * from './util/convertBytesLikeToArrayBuffer.js';
 export * from './util/convertBytesLikeToUint8Array.js';
diff --git a/packages/common/src/time/ServerDateTime.ts b/packages/common/src/time/ServerDateTime.ts
new file mode 100644
index 00000000..ea9c214d
--- /dev/null
+++ b/packages/common/src/time/ServerDateTime.ts
@@ -0,0 +1,158 @@
+const SERVER_UTC_OFFSET_MINUTES = 9 * 60;
+const SERVER_UTC_OFFSET_MS = SERVER_UTC_OFFSET_MINUTES * 60_000;
+
+export type ServerDateTimeFormat =
+    | 'dateTimeSeconds'
+    | 'dateTimeMinutes'
+    | 'date'
+    | 'timeSeconds'
+    | 'hourMinute'
+    | 'minuteSecond'
+    | 'monthDayTime'
+    | 'monthDayTimeSeconds';
+
+export type ServerDateTimeOptions = {
+    format?: ServerDateTimeFormat;
+    fallback?: string;
+};
+
+type DateTimeParts = {
+    year: number;
+    month: number;
+    day: number;
+    hour: number;
+    minute: number;
+    second: number;
+    millisecond: number;
+};
+
+const SERVER_WALL_TIME_PATTERN = /^(\d{4,6})-(\d{2})-(\d{2})(?:[ T](\d{2}):(\d{2})(?::(\d{2})(?:\.(\d{1,3}))?)?)?$/u;
+
+const pad = (value: number, length = 2): string => String(value).padStart(length, '0');
+
+const isValidParts = (parts: DateTimeParts): boolean => {
+    const candidate = new Date(0);
+    candidate.setUTCFullYear(parts.year, parts.month - 1, parts.day);
+    candidate.setUTCHours(parts.hour, parts.minute, parts.second, parts.millisecond);
+    return (
+        candidate.getUTCFullYear() === parts.year &&
+        candidate.getUTCMonth() + 1 === parts.month &&
+        candidate.getUTCDate() === parts.day &&
+        candidate.getUTCHours() === parts.hour &&
+        candidate.getUTCMinutes() === parts.minute &&
+        candidate.getUTCSeconds() === parts.second &&
+        candidate.getUTCMilliseconds() === parts.millisecond
+    );
+};
+
+const parseServerWallTime = (value: string): DateTimeParts | null => {
+    const match = SERVER_WALL_TIME_PATTERN.exec(value.trim());
+    if (!match) {
+        return null;
+    }
+    const millisecondText = match[7] ?? '';
+    const parts: DateTimeParts = {
+        year: Number(match[1]),
+        month: Number(match[2]),
+        day: Number(match[3]),
+        hour: Number(match[4] ?? 0),
+        minute: Number(match[5] ?? 0),
+        second: Number(match[6] ?? 0),
+        millisecond: Number(millisecondText.padEnd(3, '0')),
+    };
+    return isValidParts(parts) ? parts : null;
+};
+
+const partsFromInstant = (value: string | Date): DateTimeParts | null => {
+    const date = value instanceof Date ? new Date(value.getTime()) : new Date(value);
+    if (Number.isNaN(date.getTime())) {
+        return null;
+    }
+    const shifted = new Date(date.getTime() + SERVER_UTC_OFFSET_MS);
+    return {
+        year: shifted.getUTCFullYear(),
+        month: shifted.getUTCMonth() + 1,
+        day: shifted.getUTCDate(),
+        hour: shifted.getUTCHours(),
+        minute: shifted.getUTCMinutes(),
+        second: shifted.getUTCSeconds(),
+        millisecond: shifted.getUTCMilliseconds(),
+    };
+};
+
+const resolveParts = (value: string | Date): DateTimeParts | null => {
+    if (typeof value === 'string') {
+        const wallTime = parseServerWallTime(value);
+        if (wallTime) {
+            return wallTime;
+        }
+    }
+    return partsFromInstant(value);
+};
+
+const formatParts = (parts: DateTimeParts, format: ServerDateTimeFormat): string => {
+    const year = pad(parts.year, 4);
+    const month = pad(parts.month);
+    const day = pad(parts.day);
+    const hour = pad(parts.hour);
+    const minute = pad(parts.minute);
+    const second = pad(parts.second);
+
+    switch (format) {
+        case 'dateTimeMinutes':
+            return `${year}-${month}-${day} ${hour}:${minute}`;
+        case 'date':
+            return `${year}-${month}-${day}`;
+        case 'timeSeconds':
+            return `${hour}:${minute}:${second}`;
+        case 'hourMinute':
+            return `${hour}:${minute}`;
+        case 'minuteSecond':
+            return `${minute}:${second}`;
+        case 'monthDayTime':
+            return `${month}-${day} ${hour}:${minute}`;
+        case 'monthDayTimeSeconds':
+            return `${month}-${day} ${hour}:${minute}:${second}`;
+        case 'dateTimeSeconds':
+        default:
+            return `${year}-${month}-${day} ${hour}:${minute}:${second}`;
+    }
+};
+
+/**
+ * Formats an instant in the service's fixed UTC+9 wall clock.
+ *
+ * Timezone-less legacy DATETIME strings are already server wall-clock values and
+ * therefore keep their components. This deliberate fixed offset also avoids
+ * historical IANA timezone rules changing ancient in-game years.
+ */
+export const formatServerDateTime = (
+    value: string | Date | null | undefined,
+    options: ServerDateTimeOptions = {}
+): string => {
+    if (value === null || value === undefined || value === '') {
+        return options.fallback ?? '';
+    }
+    const parts = resolveParts(value);
+    if (!parts) {
+        return options.fallback ?? String(value);
+    }
+    return formatParts(parts, options.format ?? 'dateTimeSeconds');
+};
+
+export const toServerDateTimeInputValue = (value: string | Date | null | undefined): string => {
+    const formatted = formatServerDateTime(value, { format: 'dateTimeMinutes', fallback: '' });
+    return formatted ? formatted.replace(' ', 'T') : '';
+};
+
+/** Converts an HTML datetime-local value, interpreted as UTC+9 server wall time, to ISO UTC. */
+export const serverDateTimeInputToIso = (value: string): string | undefined => {
+    const parts = parseServerWallTime(value);
+    if (!parts) {
+        return undefined;
+    }
+    const wallTime = new Date(0);
+    wallTime.setUTCFullYear(parts.year, parts.month - 1, parts.day);
+    wallTime.setUTCHours(parts.hour, parts.minute, parts.second, parts.millisecond);
+    return new Date(wallTime.getTime() - SERVER_UTC_OFFSET_MS).toISOString();
+};
diff --git a/packages/common/test/server-date-time.test.ts b/packages/common/test/server-date-time.test.ts
new file mode 100644
index 00000000..8556ed3a
--- /dev/null
+++ b/packages/common/test/server-date-time.test.ts
@@ -0,0 +1,44 @@
+import { describe, expect, it } from 'vitest';
+
+import {
+    formatServerDateTime,
+    serverDateTimeInputToIso,
+    toServerDateTimeInputValue,
+} from '../src/time/ServerDateTime.js';
+
+describe('formatServerDateTime', () => {
+    it('formats ISO instants with the fixed UTC+9 service offset', () => {
+        expect(formatServerDateTime('2026-08-13T00:05:06.000Z')).toBe('2026-08-13 09:05:06');
+        expect(formatServerDateTime('0185-01-02T00:04:05.000Z')).toBe('0185-01-02 09:04:05');
+        expect(formatServerDateTime('2026-08-13T18:05:06.000Z', { format: 'date' })).toBe('2026-08-14');
+        expect(formatServerDateTime('2026-08-13T18:05:06.000Z', { format: 'hourMinute' })).toBe('03:05');
+    });
+
+    it('preserves timezone-less legacy wall-clock values', () => {
+        expect(formatServerDateTime('0185-01-02 03:04:05')).toBe('0185-01-02 03:04:05');
+        expect(formatServerDateTime('0185-01-02T03:04:05', { format: 'monthDayTime' })).toBe('01-02 03:04');
+        expect(formatServerDateTime('2026-08-13 09:05:06', { format: 'minuteSecond' })).toBe('05:06');
+    });
+
+    it('offers explicit shapes and predictable fallbacks', () => {
+        const value = '2026-08-13T00:05:06.000Z';
+        expect(formatServerDateTime(value, { format: 'dateTimeMinutes' })).toBe('2026-08-13 09:05');
+        expect(formatServerDateTime(value, { format: 'timeSeconds' })).toBe('09:05:06');
+        expect(formatServerDateTime(value, { format: 'monthDayTimeSeconds' })).toBe('08-13 09:05:06');
+        expect(formatServerDateTime(undefined, { fallback: '-' })).toBe('-');
+        expect(formatServerDateTime('not-a-date')).toBe('not-a-date');
+    });
+});
+
+describe('server datetime-local conversion', () => {
+    it('does not depend on the browser or process timezone', () => {
+        expect(serverDateTimeInputToIso('2026-08-13T09:05')).toBe('2026-08-13T00:05:00.000Z');
+        expect(toServerDateTimeInputValue('2026-08-13T00:05:00.000Z')).toBe('2026-08-13T09:05');
+    });
+
+    it('rejects invalid local input', () => {
+        expect(serverDateTimeInputToIso('2026-02-30T09:05')).toBeUndefined();
+        expect(serverDateTimeInputToIso('')).toBeUndefined();
+        expect(toServerDateTimeInputValue('not-a-date')).toBe('');
+    });
+});