diff --git a/src/components/charts/BarChart.tsx b/src/components/charts/BarChart.tsx
index 9bf3f9fc0..8eda99a25 100644
--- a/src/components/charts/BarChart.tsx
+++ b/src/components/charts/BarChart.tsx
@@ -5,7 +5,7 @@ import { ChartTooltip } from '@/components/charts/ChartTooltip';
 import { useLocale } from '@/components/hooks';
 import { renderNumberLabels } from '@/lib/charts';
 import { getThemeColors } from '@/lib/colors';
-import { DATE_FORMATS, formatDate } from '@/lib/date';
+import { DATE_FORMATS, formatDate, parseBackendDate } from '@/lib/date';
 import { formatLongCurrency, formatLongNumber } from '@/lib/format';
 
 const MemoChart = memo(Chart);
@@ -121,7 +121,7 @@ function BarChartComponent({
       const nextTooltip = opacity
         ? {
             title: formatDate(
-              new Date(dataPoints[0].raw?.d || dataPoints[0].raw?.x || dataPoints[0].raw),
+              parseBackendDate(dataPoints[0].raw?.d || dataPoints[0].raw?.x || dataPoints[0].raw),
               dateFormats[unit],
               locale,
             ),
diff --git a/src/components/hooks/useDateParameters.test.ts b/src/components/hooks/useDateParameters.test.ts
new file mode 100644
index 000000000..5bbbce187
--- /dev/null
+++ b/src/components/hooks/useDateParameters.test.ts
@@ -0,0 +1,52 @@
+import { renderHook } from '@testing-library/react';
+import { describe, expect, test, vi } from 'vitest';
+
+const useDateRangeMock = vi.fn();
+const toUtcMock = vi.fn((date: Date) => date);
+const localToUtcMock = vi.fn((date: Date) => date);
+
+vi.mock('./useDateRange', () => ({
+  useDateRange: (...args: unknown[]) => useDateRangeMock(...args),
+}));
+
+vi.mock('./useTimezone', () => ({
+  useTimezone: () => ({
+    timezone: 'America/New_York',
+    toUtc: toUtcMock,
+    localToUtc: localToUtcMock,
+    canonicalizeTimezone: (tz: string) => tz,
+  }),
+}));
+
+import { useDateParameters } from './useDateParameters';
+
+describe('useDateParameters', () => {
+  test('passes the profile timezone into useDateRange instead of ignoring it', () => {
+    useDateRangeMock.mockReturnValue({
+      dateRange: { startDate: new Date('2026-09-16T03:00:00Z'), endDate: new Date(), unit: 'hour' },
+    });
+
+    renderHook(() => useDateParameters());
+
+    // Regression test: this used to call useDateRange() with no arguments at all,
+    // silently computing "today" from the browser's own system clock instead of
+    // the profile timezone setting.
+    expect(useDateRangeMock).toHaveBeenCalledWith({ timezone: 'America/New_York' });
+  });
+
+  test('converts using the profile timezone, not the browser-local one', () => {
+    const startDate = new Date('2026-09-16T03:00:00Z');
+    const endDate = new Date('2026-09-17T03:00:00Z');
+    useDateRangeMock.mockReturnValue({ dateRange: { startDate, endDate, unit: 'day' } });
+    toUtcMock.mockClear();
+    localToUtcMock.mockClear();
+
+    renderHook(() => useDateParameters());
+
+    // Regression test: this used to call localToUtc (browser-system-timezone-aware)
+    // instead of toUtc (profile-timezone-aware) to produce the final startAt/endAt.
+    expect(toUtcMock).toHaveBeenCalledWith(startDate);
+    expect(toUtcMock).toHaveBeenCalledWith(endDate);
+    expect(localToUtcMock).not.toHaveBeenCalled();
+  });
+});
diff --git a/src/components/hooks/useDateParameters.ts b/src/components/hooks/useDateParameters.ts
index a16949017..2003be62a 100644
--- a/src/components/hooks/useDateParameters.ts
+++ b/src/components/hooks/useDateParameters.ts
@@ -10,13 +10,13 @@ function safeToISOString(date: Date, fallback: Date): string {
 }
 
 export function useDateParameters() {
+  const { timezone, toUtc, canonicalizeTimezone } = useTimezone();
   const {
     dateRange: { startDate, endDate, unit },
-  } = useDateRange();
-  const { timezone, localToUtc, canonicalizeTimezone } = useTimezone();
+  } = useDateRange({ timezone });
 
-  const utcStart = localToUtc(startDate);
-  const utcEnd = localToUtc(endDate);
+  const utcStart = toUtc(startDate);
+  const utcEnd = toUtc(endDate);
   const startAtValue = +utcStart;
   const endAtValue = +utcEnd;
   const isStartValid = Number.isFinite(startAtValue);
diff --git a/src/lib/clickhouse.test.ts b/src/lib/clickhouse.test.ts
index 1fcdedd5f..6c8c7e0b2 100644
--- a/src/lib/clickhouse.test.ts
+++ b/src/lib/clickhouse.test.ts
@@ -1,5 +1,5 @@
 import { describe, expect, test } from 'vitest';
-import { CLICKHOUSE_DATE_FORMATS } from './clickhouse';
+import clickhouse, { CLICKHOUSE_DATE_FORMATS } from './clickhouse';
 
 describe('CLICKHOUSE_DATE_FORMATS', () => {
   test('uses date format tokens compatible with ClickHouse 22.8 and newer', () => {
@@ -10,3 +10,32 @@ describe('CLICKHOUSE_DATE_FORMATS', () => {
     });
   });
 });
+
+describe('getDateSQL bucket formatting', () => {
+  // Regression: getDateSQL used to return a native DateTime whose "iso" serialization
+  // is a genuine UTC instant - unlike Postgres's already-shifted-but-Z-labeled value,
+  // which the frontend assumes for every backend. formatDateTime makes it match.
+  test('formats an explicit "Z" via formatDateTime instead of relying on native DateTime serialization', () => {
+    expect(clickhouse.getDateSQL('website_event.created_at', 'hour', 'Asia/Tokyo')).toBe(
+      `formatDateTime(date_trunc('hour', website_event.created_at, 'Asia/Tokyo'), '%Y-%m-%dT%H:00:00Z', 'Asia/Tokyo')`,
+    );
+  });
+
+  test('omits the timezone argument to formatDateTime when no timezone is given', () => {
+    expect(clickhouse.getDateSQL('website_event.created_at', 'day')).toBe(
+      `formatDateTime(date_trunc('day', website_event.created_at), '%Y-%m-%dT00:00:00Z')`,
+    );
+  });
+
+  test('covers every bucket unit with an explicit "Z"', () => {
+    expect(clickhouse.getDateSQL('created_at', 'minute', 'UTC')).toBe(
+      `formatDateTime(date_trunc('minute', created_at, 'UTC'), '%Y-%m-%dT%R:00Z', 'UTC')`,
+    );
+    expect(clickhouse.getDateSQL('created_at', 'month', 'UTC')).toBe(
+      `formatDateTime(date_trunc('month', created_at, 'UTC'), '%Y-%m-01T00:00:00Z', 'UTC')`,
+    );
+    expect(clickhouse.getDateSQL('created_at', 'year', 'UTC')).toBe(
+      `formatDateTime(date_trunc('year', created_at, 'UTC'), '%Y-01-01T00:00:00Z', 'UTC')`,
+    );
+  });
+});
diff --git a/src/lib/clickhouse.ts b/src/lib/clickhouse.ts
index d5fdbc10f..c26444c1d 100644
--- a/src/lib/clickhouse.ts
+++ b/src/lib/clickhouse.ts
@@ -18,6 +18,17 @@ export const CLICKHOUSE_DATE_FORMATS = {
   year: '%Y-01-01',
 };
 
+// Always Z-suffixed so JS parses the already-shifted local value as an unambiguous
+// instant instead of re-interpreting it in the runtime's own timezone. Matches
+// Postgres's getDateSQL convention - see prisma.ts's DATE_FORMATS.
+const CLICKHOUSE_BUCKET_FORMATS = {
+  minute: '%Y-%m-%dT%R:00Z',
+  hour: '%Y-%m-%dT%H:00:00Z',
+  day: '%Y-%m-%dT00:00:00Z',
+  month: '%Y-%m-01T00:00:00Z',
+  year: '%Y-01-01T00:00:00Z',
+};
+
 const log = debug('umami:clickhouse');
 
 const EQUALITY_OPERATORS: Operator[] = [OPERATORS.equals, OPERATORS.notEquals];
@@ -89,11 +100,12 @@ function getDateStringSQL(data: any, unit: string = 'utc', timezone?: string) {
 
 function getDateSQL(field: string, unit: string, timezone?: string) {
   const tz = normalizeTimezone(timezone);
+  const format = CLICKHOUSE_BUCKET_FORMATS[unit];
 
   if (tz) {
-    return `toDateTime(date_trunc('${unit}', ${field}, '${tz}'), '${tz}')`;
+    return `formatDateTime(date_trunc('${unit}', ${field}, '${tz}'), '${format}', '${tz}')`;
   }
-  return `toDateTime(date_trunc('${unit}', ${field}))`;
+  return `formatDateTime(date_trunc('${unit}', ${field}), '${format}')`;
 }
 
 function getSearchSQL(column: string, param: string = 'search'): string {
diff --git a/src/lib/date.test.ts b/src/lib/date.test.ts
index 0fa818d20..12b6982cf 100644
--- a/src/lib/date.test.ts
+++ b/src/lib/date.test.ts
@@ -408,6 +408,47 @@ describe('generateTimeSeries', () => {
     expect(series).toHaveLength(1);
     expect(series[0].y).toBeNull();
   });
+
+  test('matches backend-bucketed values without re-shifting them through the system zone', () => {
+    // Backend x is already the target-zone wall clock (e.g. Tokyo's day bucket),
+    // stamped with a literal "Z". minDate/maxDate carry that same wall clock via
+    // their local getters, per the "fake zoned" Date convention from parseDateRange.
+    const minDate = new Date(2026, 6, 2); // local wall clock: 2026-07-02 00:00
+    const data = [{ x: '2026-07-02T00:00:00Z', y: 42 }];
+
+    const series = generateTimeSeries(data, minDate, minDate, 'day', 'en-US');
+
+    expect(series).toHaveLength(1);
+    expect(series[0].x).toBe('2026-07-02');
+    expect(series[0].y).toBe(42);
+  });
+
+  test('does not double-shift hour buckets near a day boundary', () => {
+    // Regression guard: an earlier version of this fix re-applied toZonedTime(x, timezone)
+    // on top of the backend's already-shifted value, pushing a 23:00 bucket into the
+    // next day. The backend value must be read back as-is (via 'UTC'), not re-shifted.
+    const minDate = new Date(2026, 6, 2, 23); // local wall clock: 2026-07-02 23:00
+    const data = [{ x: '2026-07-02T23:00:00Z', y: 7 }];
+
+    const series = generateTimeSeries(data, minDate, minDate, 'hour', 'en-US');
+
+    expect(series).toHaveLength(1);
+    expect(series[0].x).toBe('2026-07-02 23');
+    expect(series[0].y).toBe(7);
+  });
+
+  test('does not reinterpret plain local bucket keys (no "Z") as UTC', () => {
+    // Regression guard: PropertyDateChart's bucket keys have no "Z" (plain local
+    // strings) and must be read as-is, not run through the "fake-Z" reinterpretation.
+    const minDate = new Date(2026, 6, 2, 23); // local wall clock: 2026-07-02 23:00
+    const data = [{ x: '2026-07-02T23:00:00', y: 7 }];
+
+    const series = generateTimeSeries(data, minDate, minDate, 'hour', 'en-US');
+
+    expect(series).toHaveLength(1);
+    expect(series[0].x).toBe('2026-07-02 23');
+    expect(series[0].y).toBe(7);
+  });
 });
 
 describe('getDateRangeValue / getMonthDateRangeValue', () => {
diff --git a/src/lib/date.ts b/src/lib/date.ts
index 82fba23f8..0b8bc2ca4 100644
--- a/src/lib/date.ts
+++ b/src/lib/date.ts
@@ -350,6 +350,15 @@ export function formatDate(
   });
 }
 
+// Backend bucket values are already zone-shifted and stamped with a literal "Z",
+// so they must be read back via UTC getters, not the browser's own zone. Values
+// without a "Z" (e.g. PropertyDateChart's own bucket keys) are read as-is.
+export function parseBackendDate(value: string | number | Date): Date {
+  return typeof value === 'string' && value.endsWith('Z')
+    ? toZonedTime(new Date(value), 'UTC')
+    : new Date(value);
+}
+
 export function generateTimeSeries(
   data: { x: string; y: number; d?: string }[],
   minDate: Date,
@@ -371,7 +380,9 @@ export function generateTimeSeries(
     current = add(current, 1);
   }
 
-  const lookup = new Map(data.map(({ x, y, d }) => [formatDate(x, fmt, locale), { x, y, d }]));
+  const lookup = new Map(
+    data.map(({ x, y, d }) => [formatDate(parseBackendDate(x), fmt, locale), { x, y, d }]),
+  );
 
   return timeseries.map(t => {
     const { x, y, d } = lookup.get(t) || {};
diff --git a/src/lib/prisma.test.ts b/src/lib/prisma.test.ts
index cbb417bea..8052c79e6 100644
--- a/src/lib/prisma.test.ts
+++ b/src/lib/prisma.test.ts
@@ -82,6 +82,54 @@ describe('getRawQueryClient', () => {
   });
 });
 
+describe('getDateSQL timezone formatting', () => {
+  test('formats UTC (default) buckets with an explicit zone and a Z marker', () => {
+    // Never rely on the DB session's ambient timezone.
+    expect(prisma.getDateSQL('website_event.created_at', 'hour')).toBe(
+      `to_char(date_trunc('hour', website_event.created_at at time zone 'UTC'), 'YYYY-MM-DD"T"HH24:00:00"Z"')`,
+    );
+    expect(prisma.getDateSQL('website_event.created_at', 'hour', 'UTC')).toBe(
+      `to_char(date_trunc('hour', website_event.created_at at time zone 'UTC'), 'YYYY-MM-DD"T"HH24:00:00"Z"')`,
+    );
+  });
+
+  test('formats non-UTC buckets with the same explicit Z marker, not a bare timestamp', () => {
+    // Bare timestamps get misparsed as local-to-the-runtime, not the source zone.
+    expect(prisma.getDateSQL('website_event.created_at', 'hour', 'Asia/Tehran')).toBe(
+      `to_char(date_trunc('hour', website_event.created_at at time zone 'Asia/Tehran'), 'YYYY-MM-DD"T"HH24:00:00"Z"')`,
+    );
+  });
+});
+
+describe('getDateStringSQL timezone formatting', () => {
+  test('formats non-UTC second-precision values with an explicit Z marker', () => {
+    expect(prisma.getDateStringSQL('event_data.date_value', 'second', 'Asia/Tehran')).toBe(
+      `to_char(event_data.date_value at time zone 'Asia/Tehran', 'YYYY-MM-DD"T"HH24:MI:SS"Z"')`,
+    );
+  });
+
+  test('defaults to explicit UTC formatting regardless of unit when no timezone is given', () => {
+    expect(prisma.getDateStringSQL('event_data.date_value')).toBe(
+      `to_char(event_data.date_value at time zone 'UTC', 'YYYY-MM-DD"T"HH24:MI:SS"Z"')`,
+    );
+  });
+});
+
+describe('getDateWeeklySQL timezone formatting', () => {
+  test('falls back to UTC instead of producing invalid SQL when no timezone is given', () => {
+    // Regression test: this used to interpolate `undefined` straight into `at time zone`.
+    expect(prisma.getDateWeeklySQL('website_event.created_at')).toBe(
+      `concat(extract(dow from (website_event.created_at at time zone 'UTC')), ':', to_char((website_event.created_at at time zone 'UTC'), 'HH24'))`,
+    );
+  });
+
+  test('uses the given timezone when one is provided', () => {
+    expect(prisma.getDateWeeklySQL('website_event.created_at', 'Asia/Tehran')).toBe(
+      `concat(extract(dow from (website_event.created_at at time zone 'Asia/Tehran')), ':', to_char((website_event.created_at at time zone 'Asia/Tehran'), 'HH24'))`,
+    );
+  });
+});
+
 describe('pagedRawQuery default ordering', () => {
   function mockQueries(count = '4') {
     const queryRaw = vi.mocked((prisma.client as any).$queryRawUnsafe);
diff --git a/src/lib/prisma.ts b/src/lib/prisma.ts
index 11dc2a189..d07df4bf3 100644
--- a/src/lib/prisma.ts
+++ b/src/lib/prisma.ts
@@ -63,15 +63,9 @@ export function getRawQueryClient(
   return client;
 }
 
+// Always Z-suffixed so JS parses the already-shifted local value as an unambiguous
+// instant instead of re-interpreting it in the runtime's own timezone.
 const DATE_FORMATS = {
-  minute: 'YYYY-MM-DD HH24:MI:00',
-  hour: 'YYYY-MM-DD HH24:00:00',
-  day: 'YYYY-MM-DD HH24:00:00',
-  month: 'YYYY-MM-01 HH24:00:00',
-  year: 'YYYY-01-01 HH24:00:00',
-};
-
-const DATE_FORMATS_UTC = {
   minute: 'YYYY-MM-DD"T"HH24:MI:00"Z"',
   hour: 'YYYY-MM-DD"T"HH24:00:00"Z"',
   day: 'YYYY-MM-DD"T"HH24:00:00"Z"',
@@ -81,7 +75,7 @@ const DATE_FORMATS_UTC = {
 
 const DATE_STRING_FORMATS = {
   utc: 'YYYY-MM-DD"T"HH24:MI:SS"Z"',
-  second: 'YYYY-MM-DD"T"HH24:MI:SS',
+  second: 'YYYY-MM-DD"T"HH24:MI:SS"Z"',
 };
 
 function isUtcTimezone(timezone?: string) {
@@ -101,11 +95,11 @@ function getCastColumnQuery(field: string, type: string): string {
 }
 
 function getDateSQL(field: string, unit: string, timezone?: string): string {
-  if (timezone && !isUtcTimezone(timezone)) {
-    return `to_char(date_trunc('${unit}', ${field} at time zone '${timezone}'), '${DATE_FORMATS[unit]}')`;
-  }
+  // Explicit `at time zone` even for UTC — `date_trunc` without one falls back to the
+  // DB session's ambient TimeZone setting, which this app never pins.
+  const tz = timezone && !isUtcTimezone(timezone) ? timezone : 'UTC';
 
-  return `to_char(date_trunc('${unit}', ${field}), '${DATE_FORMATS_UTC[unit]}')`;
+  return `to_char(date_trunc('${unit}', ${field} at time zone '${tz}'), '${DATE_FORMATS[unit]}')`;
 }
 
 function getDateStringSQL(
@@ -113,15 +107,17 @@ function getDateStringSQL(
   unit: keyof typeof DATE_STRING_FORMATS = 'utc',
   timezone?: string,
 ): string {
-  if (timezone && !isUtcTimezone(timezone)) {
-    return `to_char(${field} at time zone '${timezone}', '${DATE_STRING_FORMATS[unit]}')`;
-  }
+  const isUtc = !timezone || isUtcTimezone(timezone);
+  const tz = isUtc ? 'UTC' : timezone;
+  const format = isUtc ? DATE_STRING_FORMATS.utc : DATE_STRING_FORMATS[unit];
 
-  return `to_char(${field}, '${DATE_STRING_FORMATS.utc}')`;
+  return `to_char(${field} at time zone '${tz}', '${format}')`;
 }
 
 function getDateWeeklySQL(field: string, timezone?: string) {
-  return `concat(extract(dow from (${field} at time zone '${timezone}')), ':', to_char((${field} at time zone '${timezone}'), 'HH24'))`;
+  const tz = timezone && !isUtcTimezone(timezone) ? timezone : 'UTC';
+
+  return `concat(extract(dow from (${field} at time zone '${tz}')), ':', to_char((${field} at time zone '${tz}'), 'HH24'))`;
 }
 
 export function getTimestampSQL(field: string) {
diff --git a/src/lib/request.test.ts b/src/lib/request.test.ts
index a23550dec..9a955e214 100644
--- a/src/lib/request.test.ts
+++ b/src/lib/request.test.ts
@@ -3,6 +3,11 @@ import { fetchWebsite } from '@/lib/load';
 import { getWebsiteSegment } from '@/queries/prisma';
 import { getQueryFilters } from './request';
 
+vi.hoisted(() => {
+  process.env.DATABASE_URL ??= 'postgresql://user:pass@localhost:5432/umami?schema=public';
+  delete process.env.DATABASE_REPLICA_URL;
+});
+
 vi.mock('@/lib/load', () => ({
   fetchAccount: vi.fn(),
   fetchWebsite: vi.fn(),
diff --git a/src/queries/sql/retention/getRetention.ts b/src/queries/sql/retention/getRetention.ts
index a48710381..874dad604 100644
--- a/src/queries/sql/retention/getRetention.ts
+++ b/src/queries/sql/retention/getRetention.ts
@@ -133,7 +133,10 @@ async function clickhouseQuery(
     user_activities AS (
       select distinct
         website_event.session_id as session_id,
-        toInt32((${getDateSQL('created_at', unit, timezone)} - cohort_items.cohort_date) / 86400) as day_number
+        toInt32(dateDiff('day',
+          toDate(parseDateTimeBestEffort(cohort_items.cohort_date)),
+          toDate(parseDateTimeBestEffort(${getDateSQL('created_at', unit, timezone)}))
+        )) as day_number
       from website_event
       join cohort_items
       on website_event.session_id = cohort_items.session_id