diff --git a/app/libs/date-utils.ts b/app/libs/date-utils.ts
index a6c9dd77..47e56d29 100644
--- a/app/libs/date-utils.ts
+++ b/app/libs/date-utils.ts
@@ -1,5 +1,23 @@
 import dayjs from '~/app/libs/dayjs'
 
+/**
+ * 組織タイムゾーン基準で N ヶ月前の日付境界を UTC ISO 文字列で返す。
+ * 'all' の場合はエポック相当の固定値を返す。
+ */
+export function calcSinceDate(
+  periodMonths: number | 'all',
+  timezone: string,
+): string {
+  if (periodMonths === 'all') return '2000-01-01T00:00:00.000Z'
+  return dayjs
+    .utc()
+    .tz(timezone)
+    .subtract(periodMonths, 'month')
+    .startOf('day')
+    .utc()
+    .toISOString()
+}
+
 export const parseDate = (date: string | null, timeZone: string) => {
   const dt = date ? dayjs(date, 'YYYY-MM-DD') : dayjs()
   return dt.tz(timeZone).startOf('day')
diff --git a/app/routes/$orgSlug/analysis/feedbacks/_index/index.tsx b/app/routes/$orgSlug/analysis/feedbacks/_index/index.tsx
index 049553f2..30342166 100644
--- a/app/routes/$orgSlug/analysis/feedbacks/_index/index.tsx
+++ b/app/routes/$orgSlug/analysis/feedbacks/_index/index.tsx
@@ -29,8 +29,8 @@ import {
   TableHeader,
   TableRow,
 } from '~/app/components/ui/table'
-import dayjs from '~/app/libs/dayjs'
-import { orgContext } from '~/app/middleware/context'
+import { calcSinceDate } from '~/app/libs/date-utils'
+import { orgContext, timezoneContext } from '~/app/middleware/context'
 import { listTeams } from '~/app/routes/$orgSlug/settings/teams._index/queries.server'
 import { DataTablePagination } from './+components/data-table-pagination'
 import { feedbackColumns } from './+components/feedback-columns'
@@ -60,6 +60,7 @@ const VALID_PERIODS = [1, 3, 6, 12]
 
 export const loader = async ({ request, context }: Route.LoaderArgs) => {
   const { organization } = context.get(orgContext)
+  const timezone = context.get(timezoneContext)
 
   const url = new URL(request.url)
   const teamParam = url.searchParams.get('team') || undefined
@@ -71,10 +72,7 @@ export const loader = async ({ request, context }: Route.LoaderArgs) => {
         ? Number(periodParam)
         : 1
 
-  const sinceDate =
-    periodMonths === 'all'
-      ? '2000-01-01T00:00:00.000Z'
-      : dayjs.utc().subtract(periodMonths, 'month').startOf('day').toISOString()
+  const sinceDate = calcSinceDate(periodMonths, timezone)
 
   const page = Number(url.searchParams.get('page') || '1')
   const perPage = Number(url.searchParams.get('per_page') || '20')
diff --git a/app/routes/$orgSlug/analysis/inventory/index.tsx b/app/routes/$orgSlug/analysis/inventory/index.tsx
index bc33f245..1549892d 100644
--- a/app/routes/$orgSlug/analysis/inventory/index.tsx
+++ b/app/routes/$orgSlug/analysis/inventory/index.tsx
@@ -16,6 +16,7 @@ import {
   SelectValue,
 } from '~/app/components/ui/select'
 import { Switch } from '~/app/components/ui/switch'
+import { calcSinceDate } from '~/app/libs/date-utils'
 import dayjs from '~/app/libs/dayjs'
 import { orgContext, timezoneContext } from '~/app/middleware/context'
 import { listTeams } from '~/app/routes/$orgSlug/settings/teams._index/queries.server'
@@ -54,13 +55,7 @@ export const loader = async ({ request, context }: Route.LoaderArgs) => {
   const excludeBots = url.searchParams.get('excludeBots') !== '0'
   const unreviewedOnly = url.searchParams.get('unreviewedOnly') === '1'
 
-  const sinceDate = dayjs
-    .utc()
-    .tz(timezone)
-    .subtract(periodMonths, 'month')
-    .startOf('day')
-    .utc()
-    .toISOString()
+  const sinceDate = calcSinceDate(periodMonths, timezone)
 
   const now = dayjs.utc().toISOString()
 
diff --git a/app/routes/$orgSlug/analysis/reviews/index.tsx b/app/routes/$orgSlug/analysis/reviews/index.tsx
index e4edb7bc..34817f80 100644
--- a/app/routes/$orgSlug/analysis/reviews/index.tsx
+++ b/app/routes/$orgSlug/analysis/reviews/index.tsx
@@ -15,8 +15,8 @@ import {
   SelectValue,
 } from '~/app/components/ui/select'
 import { Stack } from '~/app/components/ui/stack'
-import dayjs from '~/app/libs/dayjs'
-import { orgContext } from '~/app/middleware/context'
+import { calcSinceDate } from '~/app/libs/date-utils'
+import { orgContext, timezoneContext } from '~/app/middleware/context'
 import { listTeams } from '~/app/routes/$orgSlug/settings/teams._index/queries.server'
 import { getOrgCachedData } from '~/app/services/cache.server'
 import { PRSizeChart } from './+components/pr-size-chart'
@@ -50,6 +50,7 @@ const PERIOD_OPTIONS = [
 
 export const loader = async ({ request, context }: Route.LoaderArgs) => {
   const { organization } = context.get(orgContext)
+  const timezone = context.get(timezoneContext)
 
   const url = new URL(request.url)
   const teamParam = url.searchParams.get('team')
@@ -62,10 +63,7 @@ export const loader = async ({ request, context }: Route.LoaderArgs) => {
         ? Number(periodParam)
         : 3
 
-  const sinceDate =
-    periodMonths === 'all'
-      ? '2000-01-01T00:00:00.000Z'
-      : dayjs.utc().subtract(periodMonths, 'month').startOf('day').toISOString()
+  const sinceDate = calcSinceDate(periodMonths, timezone)
 
   const teams = await listTeams(organization.id)