diff --git a/src/components/charts/compliance-heatmap.tsx b/src/components/charts/compliance-heatmap.tsx
index 7eec7a959..cc4a0583b 100644
--- a/src/components/charts/compliance-heatmap.tsx
+++ b/src/components/charts/compliance-heatmap.tsx
@@ -1,7 +1,12 @@
 "use client";
 
 import { useState, useMemo, useEffect, useRef } from "react";
-import { useTranslations, useFormatters } from "@/lib/i18n/context";
+import {
+  useDisplayTimezone,
+  useFormatters,
+  useTranslations,
+} from "@/lib/i18n/context";
+import { heatmapDays } from "@/lib/charts/heatmap-days";
 
 interface DailyData {
   expected: number;
@@ -79,6 +84,7 @@ export function ComplianceHeatmap({
 }: ComplianceHeatmapProps) {
   const { t } = useTranslations();
   const fmt = useFormatters();
+  const timeZone = useDisplayTimezone();
   // Day keys are UTC-anchored "YYYY-MM-DD"; format at noon UTC so the
   // locale-aware renderer never lands a day off in either direction.
   const formatDay = (dateKey: string) =>
@@ -175,7 +181,6 @@ export function ComplianceHeatmap({
       t("charts.months.nov"),
       t("charts.months.dec"),
     ];
-    const now = new Date();
     const cellList: Array<{
       dateKey: string;
       col: number;
@@ -184,34 +189,18 @@ export function ComplianceHeatmap({
       data: DailyData;
     }> = [];
 
-    // Build array of dates from oldest to newest
-    const dates: Date[] = [];
-    for (let d = days - 1; d >= 0; d--) {
-      const date = new Date(now.getTime() - d * 24 * 60 * 60 * 1000);
-      dates.push(date);
-    }
-
-    // Start from the first date, align to Monday.
-    //
-    // v1.4.27 B7 / BL-P4-2 — read every weekday + month boundary off
-    // the UTC accessor pair (`getUTCDay`, `getUTCMonth`) so the
-    // computation matches the dateKey, which is also UTC-anchored
-    // (`toISOString().slice(0, 10)`). Reading in server-tz here would
-    // shift the Monday-alignment + month-marker placement when the
-    // SSR pass runs on a server in a non-Berlin timezone.
-    const firstDate = dates[0];
-    const firstDow = (firstDate.getUTCDay() + 6) % 7; // Monday = 0
+    // Oldest to newest, one entry per local day in the user's zone.
+    const dates = heatmapDays(new Date(), timeZone, days);
+    const firstDow = dates[0]?.dow ?? 0;
 
     let col = 0;
     const markers: Array<{ col: number; label: string }> = [];
     let lastMonth = -1;
 
     for (let i = 0; i < dates.length; i++) {
-      const date = dates[i];
-      const dow = (date.getUTCDay() + 6) % 7; // Monday = 0
+      const { dateKey, dow, month } = dates[i];
       const currentCol = Math.floor((i + firstDow) / 7);
       const row = dow;
-      const dateKey = date.toISOString().slice(0, 10);
       const data = dailyCompliance[dateKey] ?? {
         expected: 0,
         taken: 0,
@@ -219,7 +208,6 @@ export function ComplianceHeatmap({
       };
 
       // Track month boundaries
-      const month = date.getUTCMonth();
       if (month !== lastMonth) {
         markers.push({ col: currentCol, label: MONTH_LABELS[month] });
         lastMonth = month;
@@ -237,7 +225,7 @@ export function ComplianceHeatmap({
     }
 
     return { cells: cellList, weeks: col + 1, monthMarkers: markers };
-  }, [dailyCompliance, days, t]);
+  }, [dailyCompliance, days, t, timeZone]);
 
   const activeDays = cells.filter((cell) => cell.data.expected > 0).length;
   const summaryLabel =
diff --git a/src/components/charts/mood-heatmap.tsx b/src/components/charts/mood-heatmap.tsx
index 178064908..959d25d7e 100644
--- a/src/components/charts/mood-heatmap.tsx
+++ b/src/components/charts/mood-heatmap.tsx
@@ -1,7 +1,12 @@
 "use client";
 
 import { useState, useMemo, useEffect, useRef } from "react";
-import { useTranslations, useFormatters } from "@/lib/i18n/context";
+import {
+  useDisplayTimezone,
+  useFormatters,
+  useTranslations,
+} from "@/lib/i18n/context";
+import { heatmapDays } from "@/lib/charts/heatmap-days";
 import { moodLabelKeyForScore } from "@/lib/mood/labels";
 
 /**
@@ -62,6 +67,7 @@ export function MoodHeatmap({
 }: MoodHeatmapProps) {
   const { t } = useTranslations();
   const fmt = useFormatters();
+  const timeZone = useDisplayTimezone();
   // Day keys are UTC-anchored "YYYY-MM-DD"; format at noon UTC so the
   // locale-aware renderer never lands a day off in either direction.
   const formatDay = (dateKey: string) =>
@@ -147,7 +153,6 @@ export function MoodHeatmap({
       t("charts.months.nov"),
       t("charts.months.dec"),
     ];
-    const now = new Date();
     const cellList: Array<{
       dateKey: string;
       col: number;
@@ -156,30 +161,19 @@ export function MoodHeatmap({
       cell: MoodHeatmapCell | null;
     }> = [];
 
-    const dates: Date[] = [];
-    for (let d = days - 1; d >= 0; d--) {
-      const date = new Date(now.getTime() - d * 24 * 60 * 60 * 1000);
-      dates.push(date);
-    }
-
-    // Monday-align + month markers read off the UTC accessors so the
-    // grid matches the UTC-anchored dateKey (mirrors compliance-heatmap).
-    const firstDate = dates[0];
-    const firstDow = (firstDate.getUTCDay() + 6) % 7; // Monday = 0
+    const dates = heatmapDays(new Date(), timeZone, days);
+    const firstDow = dates[0]?.dow ?? 0;
 
     let col = 0;
     const markers: Array<{ col: number; label: string }> = [];
     let lastMonth = -1;
 
     for (let i = 0; i < dates.length; i++) {
-      const date = dates[i];
-      const dow = (date.getUTCDay() + 6) % 7; // Monday = 0
+      const { dateKey, dow, month } = dates[i];
       const currentCol = Math.floor((i + firstDow) / 7);
       const row = dow;
-      const dateKey = date.toISOString().slice(0, 10);
       const cell = cellData[dateKey] ?? null;
 
-      const month = date.getUTCMonth();
       if (month !== lastMonth) {
         markers.push({ col: currentCol, label: MONTH_LABELS[month] });
         lastMonth = month;
@@ -197,7 +191,7 @@ export function MoodHeatmap({
     }
 
     return { cells: cellList, weeks: col + 1, monthMarkers: markers };
-  }, [cellData, days, t]);
+  }, [cellData, days, t, timeZone]);
 
   const loggedDays = cells.filter((cell) => cell.cell !== null).length;
   const summaryLabel =
diff --git a/src/lib/charts/__tests__/heatmap-days.test.ts b/src/lib/charts/__tests__/heatmap-days.test.ts
new file mode 100644
index 000000000..dbc0c973b
--- /dev/null
+++ b/src/lib/charts/__tests__/heatmap-days.test.ts
@@ -0,0 +1,35 @@
+import { describe, expect, it } from "vitest";
+
+import { heatmapDays } from "../heatmap-days";
+
+describe("heatmapDays", () => {
+  it.each([
+    // 20:30 in New York is already the next UTC day.
+    ["America/New_York", "2026-03-09T00:30:00.000Z", "2026-03-08"],
+    // 08:00 in Auckland and Tonga is still the previous UTC day.
+    ["Pacific/Auckland", "2026-04-04T20:00:00.000Z", "2026-04-05"],
+    ["Pacific/Tongatapu", "2026-07-27T19:00:00.000Z", "2026-07-28"],
+    ["America/Santiago", "2026-09-06T03:30:00.000Z", "2026-09-05"],
+    ["Europe/Berlin", "2026-10-25T22:30:00.000Z", "2026-10-25"],
+  ])("ends on today's key in %s", (tz, iso, today) => {
+    const days = heatmapDays(new Date(iso), tz, 14);
+    expect(days).toHaveLength(14);
+    expect(days.at(-1)?.dateKey).toBe(today);
+    // Consecutive keys, one per day, no gap or repeat across DST.
+    for (let i = 1; i < days.length; i++) {
+      const prev = Date.parse(`${days[i - 1].dateKey}T00:00:00Z`);
+      const cur = Date.parse(`${days[i].dateKey}T00:00:00Z`);
+      expect(cur - prev).toBe(86_400_000);
+    }
+  });
+
+  it("reads weekday and month off the key", () => {
+    const [last] = heatmapDays(
+      new Date("2026-03-01T03:00:00.000Z"),
+      "America/New_York",
+      1,
+    );
+    // 22:00 on Saturday 28 February in New York.
+    expect(last).toEqual({ dateKey: "2026-02-28", dow: 5, month: 1 });
+  });
+});
diff --git a/src/lib/charts/heatmap-days.ts b/src/lib/charts/heatmap-days.ts
new file mode 100644
index 000000000..f769f767a
--- /dev/null
+++ b/src/lib/charts/heatmap-days.ts
@@ -0,0 +1,36 @@
+import { shiftDateKey, userDayKey } from "@/lib/tz/format";
+
+/** One calendar-heatmap column entry: a local day and where it sits. */
+export interface HeatmapDay {
+  /** `YYYY-MM-DD` in the user's zone. */
+  dateKey: string;
+  /** Weekday with Monday = 0 … Sunday = 6. */
+  dow: number;
+  /** Month index 0–11. */
+  month: number;
+}
+
+/**
+ * The last `days` local days in `timeZone`, oldest first, ending with today.
+ *
+ * Built from today's key in the user's zone and stepped on keys. The heatmaps
+ * used to take `now − n·24 h` and cut the key in UTC, so in the evening west
+ * of UTC (or the morning east of it) "today" was a different day from the one
+ * the server had keyed the user's readings on, and the newest cell showed the
+ * wrong day. Weekday and month come from the key itself.
+ */
+export function heatmapDays(
+  now: Date,
+  timeZone: string,
+  days: number,
+): HeatmapDay[] {
+  const today = userDayKey(now, timeZone);
+  const out: HeatmapDay[] = [];
+  for (let back = days - 1; back >= 0; back--) {
+    const dateKey = shiftDateKey(today, -back);
+    const [y, m, d] = dateKey.split("-").map(Number);
+    const utc = new Date(Date.UTC(y, m - 1, d));
+    out.push({ dateKey, dow: (utc.getUTCDay() + 6) % 7, month: m - 1 });
+  }
+  return out;
+}