diff --git a/.gitignore b/.gitignore
index db5c4cc7..7b769d1b 100644
--- a/.gitignore
+++ b/.gitignore
@@ -57,3 +57,4 @@ lab/output/
 
 # opensrc - source code for packages
 opensrc/
+AGENTS.md
diff --git a/CLAUDE.md b/CLAUDE.md
index a47fd83c..f0404d2e 100644
--- a/CLAUDE.md
+++ b/CLAUDE.md
@@ -125,6 +125,14 @@ Types are generated to `app/services/type.ts` from the database.
 
 **CamelCasePlugin と `sql` テンプレート**: `sql` テンプレートリテラル内の識別子は CamelCasePlugin で変換されない。`sql` 内でカラムを参照するときは `sql.ref('tableName.columnName')` を使うこと。
 
+### 日時・タイムゾーンの原則
+
+- **DB 保存形式**: ISO 8601（`2026-03-16T02:56:35Z`）。Z付きで保存し、ローカルタイム形式（`2026-03-16 02:56:35`）は使わない
+- **DB から読んだ日時のパース**: 必ず `dayjs.utc(value)` を使う。`dayjs(value)` はローカルタイムとして解釈されるため、タイムゾーン変換が正しく動かない
+- **表示層でのタイムゾーン変換**: `dayjs.utc(value).tz(timezone)` のパターンを使う
+- **batch での書き込み**: GitHub API から取得した ISO 8601 文字列をそのまま DB に保存する。独自のフォーマット変換をかけない
+- **`timeFormatTz`**（`batch/helper/timeformat.ts`）: レポートやスプレッドシート出力用。内部で `dayjs.utc()` を使用済み
+
 ### Path Aliases
 
 Use `~/` prefix for imports from `app/` directory:
@@ -218,7 +226,7 @@ Org-scoped tables (have `organizationId` column): `companyGithubUsers`, `exportS
 
 ### Source Code Reference
 
-Source code for dependencies is available in `opensrc/` for deeper understanding of implementation details. See `opensrc/sources.json` for the list of available packages.
+Source code for dependencies is available in `opensrc/` for deeper understanding of implementation details. See `opensrc/sources.json` for the list of available packages and their versions. Use this source code when you need to understand how a package works internally, not just its types/interface.
 
 ```bash
 npx opensrc <package>           # npm package (e.g., npx opensrc zod)
diff --git a/app/components/size-badge-popover.tsx b/app/components/size-badge-popover.tsx
index 0c909422..51aa7943 100644
--- a/app/components/size-badge-popover.tsx
+++ b/app/components/size-badge-popover.tsx
@@ -242,7 +242,7 @@ export function SizeBadgePopover({
                     </AvatarFallback>
                   </Avatar>
                 )}
-                {feedbackName} · {dayjs(feedbackAt).fromNow()}
+                {feedbackName} · {dayjs.utc(feedbackAt).fromNow()}
               </span>
             )}
             <Button
diff --git a/app/libs/business-hours.ts b/app/libs/business-hours.ts
index c8e70d39..70bcafe8 100644
--- a/app/libs/business-hours.ts
+++ b/app/libs/business-hours.ts
@@ -72,8 +72,8 @@ export const calculateBusinessHours = (
   end: string,
   timezone = 'Asia/Tokyo',
 ): number => {
-  const startDate = dayjs(start).tz(timezone)
-  const endDate = dayjs(end).tz(timezone)
+  const startDate = dayjs.utc(start).tz(timezone)
+  const endDate = dayjs.utc(end).tz(timezone)
 
   if (!startDate.isBefore(endDate)) return 0
 
@@ -127,3 +127,18 @@ export const calculateBusinessHours = (
 
   return totalHours
 }
+
+/**
+ * 2つの日時間の差を日数で返す（営業時間 or カレンダー時間）
+ */
+export const diffInDays = (
+  start: string,
+  end: string,
+  businessDaysOnly: boolean,
+  timezone = 'Asia/Tokyo',
+): number => {
+  const hours = businessDaysOnly
+    ? calculateBusinessHours(start, end, timezone)
+    : dayjs.utc(end).diff(dayjs.utc(start), 'hour', true)
+  return hours / 24
+}
diff --git a/app/routes/$orgSlug/+components/pr-block.tsx b/app/routes/$orgSlug/+components/pr-block.tsx
index b5d34658..231eb83e 100644
--- a/app/routes/$orgSlug/+components/pr-block.tsx
+++ b/app/routes/$orgSlug/+components/pr-block.tsx
@@ -77,7 +77,7 @@ function getSizeColor(complexity: string | null): BlockColor {
 }
 
 function getAgeColor(createdAt: string): BlockColor {
-  const days = dayjs().diff(dayjs(createdAt), 'day', true)
+  const days = dayjs().diff(dayjs.utc(createdAt), 'day', true)
   for (const t of AGE_THRESHOLDS) {
     if (days < t.maxDays) return { bg: t.bg, ring: t.ring, bgFaint: t.bgFaint }
   }
@@ -126,7 +126,7 @@ export function PRPopoverContent({
   showAuthor?: boolean
   reviewState?: string
 }) {
-  const ageDays = Math.floor(dayjs().diff(dayjs(pr.createdAt), 'day', true))
+  const ageDays = Math.floor(dayjs().diff(dayjs.utc(pr.createdAt), 'day', true))
   const stateInfo = reviewState ? REVIEW_STATE_STYLE[reviewState] : null
   return (
     <div className="space-y-1">
diff --git a/app/routes/$orgSlug/analysis/feedbacks/_index/+components/feedback-columns.tsx b/app/routes/$orgSlug/analysis/feedbacks/_index/+components/feedback-columns.tsx
index aa442c60..95e08204 100644
--- a/app/routes/$orgSlug/analysis/feedbacks/_index/+components/feedback-columns.tsx
+++ b/app/routes/$orgSlug/analysis/feedbacks/_index/+components/feedback-columns.tsx
@@ -132,7 +132,7 @@ export const feedbackColumns: ColumnDef<FeedbackRow, unknown>[] = [
     header: 'Updated',
     cell: (info) => (
       <span className="text-muted-foreground text-sm text-nowrap">
-        {dayjs(info.getValue()).fromNow()}
+        {dayjs.utc(info.getValue()).fromNow()}
       </span>
     ),
     enableSorting: true,
diff --git a/app/routes/$orgSlug/analysis/feedbacks/_index/index.tsx b/app/routes/$orgSlug/analysis/feedbacks/_index/index.tsx
index 2cd513e1..049553f2 100644
--- a/app/routes/$orgSlug/analysis/feedbacks/_index/index.tsx
+++ b/app/routes/$orgSlug/analysis/feedbacks/_index/index.tsx
@@ -74,7 +74,7 @@ export const loader = async ({ request, context }: Route.LoaderArgs) => {
   const sinceDate =
     periodMonths === 'all'
       ? '2000-01-01T00:00:00.000Z'
-      : dayjs().subtract(periodMonths, 'month').startOf('day').toISOString()
+      : dayjs.utc().subtract(periodMonths, 'month').startOf('day').toISOString()
 
   const page = Number(url.searchParams.get('page') || '1')
   const perPage = Number(url.searchParams.get('per_page') || '20')
diff --git a/app/routes/$orgSlug/analysis/reviews/index.tsx b/app/routes/$orgSlug/analysis/reviews/index.tsx
index a3455263..2b6ffd77 100644
--- a/app/routes/$orgSlug/analysis/reviews/index.tsx
+++ b/app/routes/$orgSlug/analysis/reviews/index.tsx
@@ -65,7 +65,7 @@ export const loader = async ({ request, context }: Route.LoaderArgs) => {
   const sinceDate =
     periodMonths === 'all'
       ? '2000-01-01T00:00:00.000Z'
-      : dayjs().subtract(periodMonths, 'month').startOf('day').toISOString()
+      : dayjs.utc().subtract(periodMonths, 'month').startOf('day').toISOString()
 
   const teams = await listTeams(organization.id)
 
diff --git a/app/routes/$orgSlug/settings/data-management/index.tsx b/app/routes/$orgSlug/settings/data-management/index.tsx
index b140947b..9f494b92 100644
--- a/app/routes/$orgSlug/settings/data-management/index.tsx
+++ b/app/routes/$orgSlug/settings/data-management/index.tsx
@@ -157,7 +157,8 @@ function RefreshSection({
         <Alert>
           <AlertDescription>
             Scheduled at{' '}
-            {dayjs(refreshRequestedAt)
+            {dayjs
+              .utc(refreshRequestedAt)
               .tz(timezone)
               .format('YYYY-MM-DD HH:mm:ss')}
             . It will run on the next crawl job.
diff --git a/app/routes/$orgSlug/settings/repositories.add/+components/repository-item.tsx b/app/routes/$orgSlug/settings/repositories.add/+components/repository-item.tsx
index 9930b287..98e48f3b 100644
--- a/app/routes/$orgSlug/settings/repositories.add/+components/repository-item.tsx
+++ b/app/routes/$orgSlug/settings/repositories.add/+components/repository-item.tsx
@@ -30,7 +30,7 @@ export const RepositoryItem = ({
       )}
       <div className="text-muted-foreground">·</div>
       <div className="text-muted-foreground text-xs">
-        {dayjs(repo.pushedAt).fromNow()}
+        {dayjs.utc(repo.pushedAt).fromNow()}
       </div>
 
       <div className="flex-1" />
diff --git a/app/routes/$orgSlug/throughput/deployed/+functions/queries.server.ts b/app/routes/$orgSlug/throughput/deployed/+functions/queries.server.ts
index 786eed05..79aff6f6 100644
--- a/app/routes/$orgSlug/throughput/deployed/+functions/queries.server.ts
+++ b/app/routes/$orgSlug/throughput/deployed/+functions/queries.server.ts
@@ -1,6 +1,5 @@
 import { pipe, sortBy } from 'remeda'
-import { calculateBusinessHours } from '~/app/libs/business-hours'
-import dayjs from '~/app/libs/dayjs'
+import { diffInDays } from '~/app/libs/business-hours'
 import { excludeBots } from '~/app/libs/tenant-query.server'
 import { getTenantDb } from '~/app/services/tenant-db.server'
 import type { OrganizationId } from '~/app/types/organization'
@@ -73,23 +72,15 @@ export const getDeployedPullRequestReport = async (
   return pipe(
     pullrequests.map((pr) => {
       const createAndDeployDiff = pr.releasedAt
-        ? (businessDaysOnly
-            ? calculateBusinessHours(
-                pr.firstCommittedAt ?? pr.pullRequestCreatedAt,
-                pr.releasedAt,
-              )
-            : dayjs(pr.releasedAt).diff(
-                dayjs(pr.firstCommittedAt ?? pr.pullRequestCreatedAt),
-                'hour',
-                true,
-              )) / 24
+        ? diffInDays(
+            pr.firstCommittedAt ?? pr.pullRequestCreatedAt,
+            pr.releasedAt,
+            businessDaysOnly,
+          )
         : null
       const deployTime =
         pr.mergedAt && pr.releasedAt
-          ? (businessDaysOnly
-              ? calculateBusinessHours(pr.mergedAt, pr.releasedAt)
-              : dayjs(pr.releasedAt).diff(dayjs(pr.mergedAt), 'hour', true)) /
-            24
+          ? diffInDays(pr.mergedAt, pr.releasedAt, businessDaysOnly)
           : null
       const achievement =
         createAndDeployDiff !== null ? createAndDeployDiff < objective : false
diff --git a/app/routes/$orgSlug/throughput/merged/+functions/queries.server.ts b/app/routes/$orgSlug/throughput/merged/+functions/queries.server.ts
index 5d7a50a6..5c2ae001 100644
--- a/app/routes/$orgSlug/throughput/merged/+functions/queries.server.ts
+++ b/app/routes/$orgSlug/throughput/merged/+functions/queries.server.ts
@@ -1,6 +1,5 @@
 import { pipe, sortBy } from 'remeda'
-import { calculateBusinessHours } from '~/app/libs/business-hours'
-import dayjs from '~/app/libs/dayjs'
+import { diffInDays } from '~/app/libs/business-hours'
 import { excludeBots } from '~/app/libs/tenant-query.server'
 import { getTenantDb } from '~/app/services/tenant-db.server'
 import type { OrganizationId } from '~/app/types/organization'
@@ -71,16 +70,11 @@ export const getMergedPullRequestReport = async (
   return pipe(
     pullrequests.map((pr) => {
       const createAndMergeDiff = pr.mergedAt
-        ? (businessDaysOnly
-            ? calculateBusinessHours(
-                pr.firstCommittedAt ?? pr.pullRequestCreatedAt,
-                pr.mergedAt,
-              )
-            : dayjs(pr.mergedAt).diff(
-                dayjs(pr.firstCommittedAt ?? pr.pullRequestCreatedAt),
-                'hour',
-                true,
-              )) / 24
+        ? diffInDays(
+            pr.firstCommittedAt ?? pr.pullRequestCreatedAt,
+            pr.mergedAt,
+            businessDaysOnly,
+          )
         : null
       const achievement =
         createAndMergeDiff !== null ? createAndMergeDiff < objective : false
diff --git a/app/routes/$orgSlug/throughput/ongoing/+functions/queries.server.ts b/app/routes/$orgSlug/throughput/ongoing/+functions/queries.server.ts
index b706f97a..097dcff1 100644
--- a/app/routes/$orgSlug/throughput/ongoing/+functions/queries.server.ts
+++ b/app/routes/$orgSlug/throughput/ongoing/+functions/queries.server.ts
@@ -1,5 +1,5 @@
 import { pipe, sortBy } from 'remeda'
-import { calculateBusinessHours } from '~/app/libs/business-hours'
+import { diffInDays } from '~/app/libs/business-hours'
 import dayjs from '~/app/libs/dayjs'
 import { excludeBots } from '~/app/libs/tenant-query.server'
 import { getTenantDb } from '~/app/services/tenant-db.server'
@@ -71,17 +71,14 @@ export const getOngoingPullRequestReport = async (
     ])
     .execute()
 
-  const now = new Date().toISOString()
+  const now = dayjs.utc().toISOString()
 
   return pipe(
     pullrequests.map((pr) => {
       const startDate = pr.firstCommittedAt ?? pr.pullRequestCreatedAt
-      const diffHours = businessDaysOnly
-        ? calculateBusinessHours(startDate, now)
-        : dayjs(now).diff(dayjs(startDate), 'hour', true)
       return {
         ...pr,
-        createAndNowDiff: diffHours / 24,
+        createAndNowDiff: diffInDays(startDate, now, businessDaysOnly),
       }
     }),
     sortBy((pr) => (pr.createAndNowDiff ? -pr.createAndNowDiff : 0)),
diff --git a/app/routes/$orgSlug/workload/$login/index.tsx b/app/routes/$orgSlug/workload/$login/index.tsx
index 01fd7f8f..f185c380 100644
--- a/app/routes/$orgSlug/workload/$login/index.tsx
+++ b/app/routes/$orgSlug/workload/$login/index.tsx
@@ -168,16 +168,19 @@ export default function MemberWeeklyPage({
       holiday: holidays[dateStr] ?? null,
       created: createdPRs.filter(
         (pr) =>
-          dayjs(pr.pullRequestCreatedAt).tz(timezone).format('YYYY-MM-DD') ===
-          dateStr,
+          dayjs
+            .utc(pr.pullRequestCreatedAt)
+            .tz(timezone)
+            .format('YYYY-MM-DD') === dateStr,
       ),
       merged: mergedPRs.filter(
         (pr) =>
-          dayjs(pr.mergedAt).tz(timezone).format('YYYY-MM-DD') === dateStr,
+          dayjs.utc(pr.mergedAt).tz(timezone).format('YYYY-MM-DD') === dateStr,
       ),
       reviewed: reviews.filter(
         (r) =>
-          dayjs(r.submittedAt).tz(timezone).format('YYYY-MM-DD') === dateStr,
+          dayjs.utc(r.submittedAt).tz(timezone).format('YYYY-MM-DD') ===
+          dateStr,
       ),
     }
   })
@@ -421,7 +424,8 @@ export default function MemberWeeklyPage({
               title={pr.title}
               url={pr.url}
               complexity={pr.complexity}
-              date={dayjs(pr.pullRequestCreatedAt)
+              date={dayjs
+                .utc(pr.pullRequestCreatedAt)
                 .tz(timezone)
                 .format('M/D HH:mm')}
             />
@@ -443,7 +447,7 @@ export default function MemberWeeklyPage({
               title={pr.title}
               url={pr.url}
               complexity={pr.complexity}
-              date={dayjs(pr.mergedAt).tz(timezone).format('M/D HH:mm')}
+              date={dayjs.utc(pr.mergedAt).tz(timezone).format('M/D HH:mm')}
               extra={pr.totalTime ? `${pr.totalTime.toFixed(1)}d` : undefined}
             />
           ))}
@@ -465,7 +469,7 @@ export default function MemberWeeklyPage({
               url={r.url}
               complexity={r.complexity}
               author={r.author}
-              date={dayjs(r.submittedAt).tz(timezone).format('M/D HH:mm')}
+              date={dayjs.utc(r.submittedAt).tz(timezone).format('M/D HH:mm')}
               reviewState={r.state}
             />
           ))}
diff --git a/app/routes/$orgSlug/workload/+components/team-stacks-chart.tsx b/app/routes/$orgSlug/workload/+components/team-stacks-chart.tsx
index 256e29f9..235aea17 100644
--- a/app/routes/$orgSlug/workload/+components/team-stacks-chart.tsx
+++ b/app/routes/$orgSlug/workload/+components/team-stacks-chart.tsx
@@ -37,7 +37,7 @@ function sortBySize(prs: StackPR[]): StackPR[] {
 // --- Age mode ---
 
 function getAgeDays(pr: StackPR): number {
-  return dayjs().diff(dayjs(pr.createdAt), 'day', true)
+  return dayjs().diff(dayjs.utc(pr.createdAt), 'day', true)
 }
 
 function sortByAge(prs: StackPR[]): StackPR[] {
diff --git a/batch/bizlogic/cycletime.ts b/batch/bizlogic/cycletime.ts
index 52d816f6..ebc5b886 100644
--- a/batch/bizlogic/cycletime.ts
+++ b/batch/bizlogic/cycletime.ts
@@ -11,7 +11,9 @@ export const codingTime = ({
 }: codingTimeProps) => {
   if (firstCommittedAt && pullRequestCreatedAt) {
     return Math.abs(
-      dayjs(pullRequestCreatedAt).diff(firstCommittedAt, 'days', true),
+      dayjs
+        .utc(pullRequestCreatedAt)
+        .diff(dayjs.utc(firstCommittedAt), 'days', true),
     )
   }
   return null
@@ -29,11 +31,15 @@ export const pickupTime = ({
 }: pickupTimeProps) => {
   if (firstReviewedAt) {
     return Math.abs(
-      dayjs(firstReviewedAt).diff(pullRequestCreatedAt, 'days', true),
+      dayjs
+        .utc(firstReviewedAt)
+        .diff(dayjs.utc(pullRequestCreatedAt), 'days', true),
     )
   }
   if (mergedAt) {
-    return Math.abs(dayjs(mergedAt).diff(pullRequestCreatedAt, 'days', true))
+    return Math.abs(
+      dayjs.utc(mergedAt).diff(dayjs.utc(pullRequestCreatedAt), 'days', true),
+    )
   }
   return null
 }
@@ -44,7 +50,9 @@ interface reviewTimeProps {
 }
 export const reviewTime = ({ firstReviewedAt, mergedAt }: reviewTimeProps) => {
   if (firstReviewedAt && mergedAt) {
-    return Math.abs(dayjs(mergedAt).diff(firstReviewedAt, 'days', true))
+    return Math.abs(
+      dayjs.utc(mergedAt).diff(dayjs.utc(firstReviewedAt), 'days', true),
+    )
   }
   return null
 }
@@ -55,7 +63,9 @@ interface deployTimeProps {
 }
 export const deployTime = ({ mergedAt, releasedAt }: deployTimeProps) => {
   if (mergedAt && releasedAt) {
-    return Math.abs(dayjs(releasedAt).diff(mergedAt, 'days', true))
+    return Math.abs(
+      dayjs.utc(releasedAt).diff(dayjs.utc(mergedAt), 'days', true),
+    )
   }
   return null
 }
@@ -83,12 +93,12 @@ export const totalTime = ({
       releasedAt,
     ],
     filter((x) => !!x),
-    sortBy((x) => dayjs(x).unix()),
+    sortBy((x) => dayjs.utc(x).unix()),
   )
   const firstTime = first(times)
   const lastTime = last(times)
   if (firstTime && lastTime) {
-    return dayjs(lastTime).diff(firstTime, 'days', true)
+    return dayjs.utc(lastTime).diff(dayjs.utc(firstTime), 'days', true)
   }
   return null
 }
diff --git a/batch/bizlogic/cycletime_codingTime.test.ts b/batch/bizlogic/cycletime_codingTime.test.ts
index 86adfc05..e6876b15 100644
--- a/batch/bizlogic/cycletime_codingTime.test.ts
+++ b/batch/bizlogic/cycletime_codingTime.test.ts
@@ -15,7 +15,7 @@ describe('codingTime', () => {
     expect(
       codingTime({
         firstCommittedAt: null,
-        pullRequestCreatedAt: '2022-08-01 10:00',
+        pullRequestCreatedAt: '2022-08-01T10:00:00Z',
       }),
     ).toBeNull()
   })
@@ -23,7 +23,7 @@ describe('codingTime', () => {
   test('null if pullRequestCreatedAt is null', () => {
     expect(
       codingTime({
-        firstCommittedAt: '2022-08-01 10:00',
+        firstCommittedAt: '2022-08-01T10:00:00Z',
         pullRequestCreatedAt: null,
       }),
     ).toBeNull()
@@ -32,8 +32,8 @@ describe('codingTime', () => {
   test('zero if same timestamp', () => {
     expect(
       codingTime({
-        firstCommittedAt: '2022-08-10 10:00',
-        pullRequestCreatedAt: '2022-08-10 10:00',
+        firstCommittedAt: '2022-08-10T10:00:00Z',
+        pullRequestCreatedAt: '2022-08-10T10:00:00Z',
       }),
     ).toStrictEqual(0)
   })
@@ -41,8 +41,8 @@ describe('codingTime', () => {
   test('diff when first committed at before than pull request created at', () => {
     expect(
       codingTime({
-        firstCommittedAt: '2022-08-01 10:00',
-        pullRequestCreatedAt: '2022-08-02 11:00',
+        firstCommittedAt: '2022-08-01T10:00:00Z',
+        pullRequestCreatedAt: '2022-08-02T11:00:00Z',
       }),
     ).toStrictEqual(1 + 1 / 24)
   })
@@ -50,8 +50,8 @@ describe('codingTime', () => {
   test('diff when first committed at after than pull request created at', () => {
     expect(
       codingTime({
-        firstCommittedAt: '2022-08-02 11:00',
-        pullRequestCreatedAt: '2022-08-01 10:00',
+        firstCommittedAt: '2022-08-02T11:00:00Z',
+        pullRequestCreatedAt: '2022-08-01T10:00:00Z',
       }),
     ).toStrictEqual(1 + 1 / 24)
   })
@@ -59,8 +59,8 @@ describe('codingTime', () => {
   test('diff one year', () => {
     expect(
       codingTime({
-        firstCommittedAt: '2022-08-01 10:00',
-        pullRequestCreatedAt: '2023-08-01 10:00',
+        firstCommittedAt: '2022-08-01T10:00:00Z',
+        pullRequestCreatedAt: '2023-08-01T10:00:00Z',
       }),
     ).toStrictEqual(365)
   })
@@ -68,8 +68,8 @@ describe('codingTime', () => {
   test('diff when after the unix time end', () => {
     expect(
       codingTime({
-        firstCommittedAt: '2022-08-01 10:00',
-        pullRequestCreatedAt: '2062-08-01 10:00',
+        firstCommittedAt: '2022-08-01T10:00:00Z',
+        pullRequestCreatedAt: '2062-08-01T10:00:00Z',
       }),
     ).toStrictEqual(40 * 365 + 10) // うるう年が10年 = +10日分ある
   })
diff --git a/batch/bizlogic/cycletime_deployTime.test.ts b/batch/bizlogic/cycletime_deployTime.test.ts
index 392b05cb..bcb60972 100644
--- a/batch/bizlogic/cycletime_deployTime.test.ts
+++ b/batch/bizlogic/cycletime_deployTime.test.ts
@@ -5,24 +5,24 @@ describe('deployTime', () => {
   test('null', () => {
     expect(deployTime({ mergedAt: null, releasedAt: null })).toBeNull()
     expect(
-      deployTime({ mergedAt: '2022-08-01 10:00', releasedAt: null }),
+      deployTime({ mergedAt: '2022-08-01T10:00:00Z', releasedAt: null }),
     ).toBeNull()
     expect(
-      deployTime({ mergedAt: null, releasedAt: '2022-08-01 10:00' }),
+      deployTime({ mergedAt: null, releasedAt: '2022-08-01T10:00:00Z' }),
     ).toBeNull()
   })
 
   test('deploy on 24 hours', () => {
     expect(
       deployTime({
-        mergedAt: '2022-08-01 10:00',
-        releasedAt: '2022-08-02 10:00',
+        mergedAt: '2022-08-01T10:00:00Z',
+        releasedAt: '2022-08-02T10:00:00Z',
       }),
     ).toStrictEqual(1)
     expect(
       deployTime({
-        mergedAt: '2022-08-02 10:00',
-        releasedAt: '2022-08-01 10:00',
+        mergedAt: '2022-08-02T10:00:00Z',
+        releasedAt: '2022-08-01T10:00:00Z',
       }),
     ).toStrictEqual(1)
   })
diff --git a/batch/bizlogic/cycletime_pickupTime.test.ts b/batch/bizlogic/cycletime_pickupTime.test.ts
index 51c760e0..9efcced7 100644
--- a/batch/bizlogic/cycletime_pickupTime.test.ts
+++ b/batch/bizlogic/cycletime_pickupTime.test.ts
@@ -5,7 +5,7 @@ describe('pickupTime', () => {
   test('null', () => {
     expect(
       pickupTime({
-        pullRequestCreatedAt: '2022-08-01 10:00',
+        pullRequestCreatedAt: '2022-08-01T10:00:00Z',
         firstReviewedAt: null,
         mergedAt: null,
       }),
@@ -15,8 +15,8 @@ describe('pickupTime', () => {
   test('reviewed at 24 hours after not merged yet', () => {
     expect(
       pickupTime({
-        pullRequestCreatedAt: '2022-08-01 10:00',
-        firstReviewedAt: '2022-08-02 10:00',
+        pullRequestCreatedAt: '2022-08-01T10:00:00Z',
+        firstReviewedAt: '2022-08-02T10:00:00Z',
         mergedAt: null,
       }),
     ).toStrictEqual(1)
@@ -25,9 +25,9 @@ describe('pickupTime', () => {
   test('merged at 24 hours after with no review', () => {
     expect(
       pickupTime({
-        pullRequestCreatedAt: '2022-08-01 10:00',
+        pullRequestCreatedAt: '2022-08-01T10:00:00Z',
         firstReviewedAt: null,
-        mergedAt: '2022-08-02 10:00',
+        mergedAt: '2022-08-02T10:00:00Z',
       }),
     ).toStrictEqual(1)
   })
@@ -35,9 +35,9 @@ describe('pickupTime', () => {
   test('reviewed at 24 hours after and merged after 48 hours', () => {
     expect(
       pickupTime({
-        pullRequestCreatedAt: '2022-08-01 10:00',
-        firstReviewedAt: '2022-08-02 10:00',
-        mergedAt: '2022-08-03 10:00',
+        pullRequestCreatedAt: '2022-08-01T10:00:00Z',
+        firstReviewedAt: '2022-08-02T10:00:00Z',
+        mergedAt: '2022-08-03T10:00:00Z',
       }),
     ).toStrictEqual(1)
   })
diff --git a/batch/bizlogic/cycletime_reviewTime.test.ts b/batch/bizlogic/cycletime_reviewTime.test.ts
index b3aa287a..b22d4bee 100644
--- a/batch/bizlogic/cycletime_reviewTime.test.ts
+++ b/batch/bizlogic/cycletime_reviewTime.test.ts
@@ -5,25 +5,25 @@ describe('reviewTime', () => {
   test('null', () => {
     expect(reviewTime({ firstReviewedAt: null, mergedAt: null })).toBeNull()
     expect(
-      reviewTime({ firstReviewedAt: '2022-08-01 10:00', mergedAt: null }),
+      reviewTime({ firstReviewedAt: '2022-08-01T10:00:00Z', mergedAt: null }),
     ).toBeNull()
     expect(
-      reviewTime({ firstReviewedAt: null, mergedAt: '2022-08-01 10:00' }),
+      reviewTime({ firstReviewedAt: null, mergedAt: '2022-08-01T10:00:00Z' }),
     ).toBeNull()
   })
 
   test('reviews 24 hours', () => {
     expect(
       reviewTime({
-        firstReviewedAt: '2022-08-01 10:00',
-        mergedAt: '2022-08-02 10:00',
+        firstReviewedAt: '2022-08-01T10:00:00Z',
+        mergedAt: '2022-08-02T10:00:00Z',
       }),
     ).toStrictEqual(1)
 
     expect(
       reviewTime({
-        firstReviewedAt: '2022-08-02 10:00',
-        mergedAt: '2022-08-01 10:00',
+        firstReviewedAt: '2022-08-02T10:00:00Z',
+        mergedAt: '2022-08-01T10:00:00Z',
       }),
     ).toStrictEqual(1)
   })
diff --git a/batch/bizlogic/cycletime_totalTime.test.ts b/batch/bizlogic/cycletime_totalTime.test.ts
index 51bddbfb..cb475ea0 100644
--- a/batch/bizlogic/cycletime_totalTime.test.ts
+++ b/batch/bizlogic/cycletime_totalTime.test.ts
@@ -6,7 +6,7 @@ describe('totalTime', () => {
     expect(
       totalTime({
         firstCommittedAt: null,
-        pullRequestCreatedAt: '2022-08-01 10:00',
+        pullRequestCreatedAt: '2022-08-01T10:00:00Z',
         firstReviewedAt: null,
         mergedAt: null,
         releasedAt: null,
@@ -17,11 +17,11 @@ describe('totalTime', () => {
   test('all specified in same', () => {
     expect(
       totalTime({
-        firstCommittedAt: '2022-08-01 10:00',
-        pullRequestCreatedAt: '2022-08-01 10:00',
-        firstReviewedAt: '2022-08-01 10:00',
-        mergedAt: '2022-08-01 10:00',
-        releasedAt: '2022-08-01 10:00',
+        firstCommittedAt: '2022-08-01T10:00:00Z',
+        pullRequestCreatedAt: '2022-08-01T10:00:00Z',
+        firstReviewedAt: '2022-08-01T10:00:00Z',
+        mergedAt: '2022-08-01T10:00:00Z',
+        releasedAt: '2022-08-01T10:00:00Z',
       }),
     ).toStrictEqual(0)
   })
@@ -29,11 +29,11 @@ describe('totalTime', () => {
   test('24 hours', () => {
     expect(
       totalTime({
-        firstCommittedAt: '2022-08-01 10:00',
-        pullRequestCreatedAt: '2022-08-02 10:00',
-        firstReviewedAt: '2022-08-01 10:00',
-        mergedAt: '2022-08-01 10:00',
-        releasedAt: '2022-08-01 10:00',
+        firstCommittedAt: '2022-08-01T10:00:00Z',
+        pullRequestCreatedAt: '2022-08-02T10:00:00Z',
+        firstReviewedAt: '2022-08-01T10:00:00Z',
+        mergedAt: '2022-08-01T10:00:00Z',
+        releasedAt: '2022-08-01T10:00:00Z',
       }),
     ).toStrictEqual(1)
   })
diff --git a/batch/db/mutations.ts b/batch/db/mutations.ts
index c18fe918..ee280fb7 100644
--- a/batch/db/mutations.ts
+++ b/batch/db/mutations.ts
@@ -2,33 +2,15 @@ import type { Insertable } from 'kysely'
 import { getTenantDb, type TenantDB } from '~/app/services/tenant-db.server'
 import type { OrganizationId } from '~/app/types/organization'
 import { logger } from '../helper/logger'
-import { timeFormatUTC } from '../helper/timeformat'
 
 export function upsertPullRequest(
   organizationId: OrganizationId,
   data: Insertable<TenantDB.PullRequests>,
 ) {
-  const firstCommittedAt = timeFormatUTC(data.firstCommittedAt)
-  const pullRequestCreatedAt = timeFormatUTC(data.pullRequestCreatedAt)
-  const firstReviewedAt = timeFormatUTC(data.firstReviewedAt)
-  const mergedAt = timeFormatUTC(data.mergedAt)
-  const closedAt = timeFormatUTC(data.closedAt)
-  const releasedAt = timeFormatUTC(data.releasedAt)
-  const updatedAt = timeFormatUTC(data.updatedAt)
-
   const tenantDb = getTenantDb(organizationId)
   return tenantDb
     .insertInto('pullRequests')
-    .values({
-      ...data,
-      firstCommittedAt,
-      pullRequestCreatedAt,
-      firstReviewedAt,
-      mergedAt,
-      closedAt,
-      releasedAt,
-      updatedAt,
-    })
+    .values(data)
     .onConflict((oc) =>
       oc.columns(['repositoryId', 'number']).doUpdateSet((eb) => ({
         repo: eb.ref('excluded.repo'),
@@ -87,20 +69,10 @@ export async function batchUpsertPullRequests(
 
   for (let i = 0; i < rows.length; i += chunkSize) {
     const chunk = rows.slice(i, i + chunkSize)
-    const values = chunk.map((data) => ({
-      ...data,
-      firstCommittedAt: timeFormatUTC(data.firstCommittedAt),
-      pullRequestCreatedAt: timeFormatUTC(data.pullRequestCreatedAt),
-      firstReviewedAt: timeFormatUTC(data.firstReviewedAt),
-      mergedAt: timeFormatUTC(data.mergedAt),
-      closedAt: timeFormatUTC(data.closedAt),
-      releasedAt: timeFormatUTC(data.releasedAt),
-      updatedAt: timeFormatUTC(data.updatedAt),
-    }))
 
     await tenantDb
       .insertInto('pullRequests')
-      .values(values)
+      .values(chunk)
       .onConflict((oc) =>
         oc.columns(['repositoryId', 'number']).doUpdateSet((eb) => ({
           repo: eb.ref('excluded.repo'),
diff --git a/batch/github/review-response.test.ts b/batch/github/review-response.test.ts
new file mode 100644
index 00000000..936d4c73
--- /dev/null
+++ b/batch/github/review-response.test.ts
@@ -0,0 +1,75 @@
+import { afterEach, beforeEach, describe, expect, it, vi } from 'vitest'
+import dayjs from '~/app/libs/dayjs'
+import type { ShapedGitHubReviewComment } from './model'
+import { analyzeReviewResponse } from './review-response'
+
+const comment = (
+  user: string,
+  createdAt: string,
+): ShapedGitHubReviewComment => ({
+  id: 1,
+  user,
+  isBot: false,
+  url: '',
+  createdAt,
+})
+
+const NOW = '2026-03-17T00:00:00Z'
+
+beforeEach(() => {
+  vi.useFakeTimers()
+  vi.setSystemTime(new Date(NOW))
+})
+afterEach(() => {
+  vi.useRealTimers()
+})
+
+const daysAgo = (n: number) => dayjs.utc(NOW).subtract(n, 'day').toISOString()
+
+describe('analyzeReviewResponse', () => {
+  it('returns empty for no comments', () => {
+    expect(analyzeReviewResponse([])).toEqual([])
+  })
+
+  it('returns empty when all comments are from the same user', () => {
+    const result = analyzeReviewResponse([
+      comment('alice', daysAgo(1)),
+      comment('alice', daysAgo(0)),
+    ])
+    expect(result).toEqual([])
+  })
+
+  it('calculates response time when reviewer changes', () => {
+    const t1 = daysAgo(2)
+    const t2 = dayjs.utc(t1).add(2, 'hour').toISOString()
+    const result = analyzeReviewResponse([
+      comment('alice', t1),
+      comment('bob', t2),
+    ])
+    expect(result).toHaveLength(1)
+    expect(result[0].author).toBe('bob')
+    expect(result[0].responseTime).toBe(2) // 2 hours
+  })
+
+  it('tracks multiple reviewer changes', () => {
+    const t1 = daysAgo(3)
+    const t2 = dayjs.utc(t1).add(1, 'hour').toISOString()
+    const t3 = dayjs.utc(t1).add(3, 'hour').toISOString()
+    const result = analyzeReviewResponse([
+      comment('alice', t1),
+      comment('bob', t2),
+      comment('alice', t3),
+    ])
+    expect(result).toHaveLength(2)
+    expect(result[0]).toMatchObject({ author: 'bob', responseTime: 1 })
+    expect(result[1]).toMatchObject({ author: 'alice', responseTime: 2 })
+  })
+
+  it('filters out comments older than 90 days', () => {
+    const result = analyzeReviewResponse([
+      comment('alice', daysAgo(91)),
+      comment('bob', daysAgo(91)),
+    ])
+    expect(result).toEqual([])
+  })
+})
diff --git a/batch/github/review-response.ts b/batch/github/review-response.ts
index 59a18866..5ff3a338 100644
--- a/batch/github/review-response.ts
+++ b/batch/github/review-response.ts
@@ -10,7 +10,9 @@ export const analyzeReviewResponse = (
 
   // 古い順に並べて、レビュアーが変わったらその時間差を反応時間として記録
   for (const res of sortBy(
-    comments.filter((d) => dayjs(d.createdAt) > dayjs().add(-90, 'days')),
+    comments.filter(
+      (d) => dayjs.utc(d.createdAt) > dayjs.utc().add(-90, 'days'),
+    ),
     [(x) => x.createdAt, 'asc'],
   )) {
     if (lastRes && lastRes.user !== res.user) {
@@ -18,7 +20,8 @@ export const analyzeReviewResponse = (
         author: res.user,
         createdAt: res.createdAt,
         responseTime:
-          (dayjs(res.createdAt).unix() - dayjs(lastRes.createdAt).unix()) /
+          (dayjs.utc(res.createdAt).unix() -
+            dayjs.utc(lastRes.createdAt).unix()) /
           60 /
           60,
       })
diff --git a/batch/helper/timeformat.test.ts b/batch/helper/timeformat.test.ts
new file mode 100644
index 00000000..1ad3928f
--- /dev/null
+++ b/batch/helper/timeformat.test.ts
@@ -0,0 +1,24 @@
+import { describe, expect, it } from 'vitest'
+import { timeFormatTz } from './timeformat'
+
+describe('timeFormatTz', () => {
+  it('converts ISO 8601 UTC to Asia/Tokyo', () => {
+    expect(timeFormatTz('2026-03-16T02:56:35Z', 'Asia/Tokyo')).toBe(
+      '2026-03-16 11:56:35',
+    )
+  })
+
+  it('converts ISO 8601 UTC to US/Pacific', () => {
+    expect(timeFormatTz('2026-03-16T02:56:35Z', 'US/Pacific')).toBe(
+      '2026-03-15 19:56:35',
+    )
+  })
+
+  it('returns null for null input', () => {
+    expect(timeFormatTz(null, 'Asia/Tokyo')).toBeNull()
+  })
+
+  it('returns null for undefined input', () => {
+    expect(timeFormatTz(undefined, 'Asia/Tokyo')).toBeNull()
+  })
+})
diff --git a/batch/helper/timeformat.ts b/batch/helper/timeformat.ts
index fccbac34..972c5511 100644
--- a/batch/helper/timeformat.ts
+++ b/batch/helper/timeformat.ts
@@ -1,16 +1,5 @@
 import dayjs from '~/app/libs/dayjs'
 
-export function timeFormatUTC<T extends string | null | undefined>(
-  date: T,
-): T extends string ? string : null {
-  if (date === null || date === undefined) {
-    return null as T extends string ? string : null
-  }
-  return dayjs(date)
-    .utc(false)
-    .format('YYYY-MM-DD HH:mm:ss') as T extends string ? string : null
-}
-
 export function timeFormatTz<T extends string | null | undefined>(
   date: T,
   tz: string,
@@ -18,7 +7,8 @@ export function timeFormatTz<T extends string | null | undefined>(
   if (date === null || date === undefined) {
     return null as T extends string ? string : null
   }
-  return dayjs(date).tz(tz).format('YYYY-MM-DD HH:mm:ss') as T extends string
-    ? string
-    : null
+  return dayjs
+    .utc(date)
+    .tz(tz)
+    .format('YYYY-MM-DD HH:mm:ss') as T extends string ? string : null
 }