diff --git a/lib/__tests__/format-relative-ago.test.ts b/lib/__tests__/format-relative-ago.test.ts
index 14d69202b..befcfba80 100644
--- a/lib/__tests__/format-relative-ago.test.ts
+++ b/lib/__tests__/format-relative-ago.test.ts
@@ -1,9 +1,15 @@
-import { describe, it, expect } from 'vitest';
+import { describe, it, expect, afterEach } from 'vitest';
 import { formatRelativeAgo } from '../utils';
+import { AUTO_TIME_ZONE } from '../timezone';
+import { useSettingsStore } from '@/stores/settings-store';
 
 describe('formatRelativeAgo', () => {
   const now = new Date('2026-07-01T12:00:00Z');
 
+  afterEach(() => {
+    useSettingsStore.setState({ timeZone: AUTO_TIME_ZONE });
+  });
+
   it('says how long ago in the largest whole unit', () => {
     expect(formatRelativeAgo(new Date('2026-07-01T11:57:00Z'), now)).toBe('3 minutes ago');
     expect(formatRelativeAgo(new Date('2026-07-01T08:00:00Z'), now)).toBe('4 hours ago');
@@ -14,4 +20,16 @@ describe('formatRelativeAgo', () => {
     expect(formatRelativeAgo(new Date('2026-05-01T12:00:00Z'), now)).toBe('');
     expect(formatRelativeAgo('not a date', now)).toBe('');
   });
+
+  it('counts calendar days in the effective time zone (#1259)', () => {
+    useSettingsStore.setState({ timeZone: 'America/Denver' });
+    // 16 Oct, 10:00 in Denver.
+    const denverNow = new Date('2026-10-16T16:00:00Z');
+    // 14 Oct, 20:51 in Denver: only 37 hours ago, but two days back.
+    expect(formatRelativeAgo(new Date('2026-10-15T02:51:30Z'), denverNow)).toBe('2 days ago');
+    // 15 Oct, 08:00 in Denver: 26 hours ago, and yesterday.
+    expect(formatRelativeAgo(new Date('2026-10-15T14:00:00Z'), denverNow)).toBe('yesterday');
+    // Under a day still reads in hours, even across midnight.
+    expect(formatRelativeAgo(new Date('2026-10-15T23:00:00Z'), denverNow)).toBe('17 hours ago');
+  });
 });
diff --git a/lib/utils.ts b/lib/utils.ts
index 6d30d4514..1af71cfcd 100644
--- a/lib/utils.ts
+++ b/lib/utils.ts
@@ -258,6 +258,10 @@ export function formatDateTime(
  * How long ago `date` was, in words ("3 minutes ago", "2 days ago"), for the
  * parenthesis Gmail puts after a message's time. Empty once it is a month
  * old, where the date alone says enough.
+ *
+ * Days are calendar days in the user's effective time zone, so a mail from
+ * the day before yesterday is never called "yesterday" just because fewer
+ * than 48 hours have passed. (#1259)
  */
 export function formatRelativeAgo(date: Date | string, now: Date = new Date()): string {
   const d = typeof date === 'string' ? new Date(date) : date;
@@ -265,7 +269,12 @@ export function formatRelativeAgo(date: Date | string, now: Date = new Date()):
   const diff = Math.max(0, now.getTime() - d.getTime());
   const minutes = Math.floor(diff / 60000);
   const hours = Math.floor(diff / 3600000);
-  const days = Math.floor(diff / 86400000);
+  const timeZone = getEffectiveTimeZone();
+  const dayNumber = (x: Date) => {
+    const w = getWallClock(x, timeZone);
+    return Date.UTC(w.year, w.month - 1, w.day) / 86400000;
+  };
+  const days = Math.max(1, dayNumber(now) - dayNumber(d));
   if (days >= 30) return '';
   const locale = getEffectiveLocale();
   const rtf = new Intl.RelativeTimeFormat(locale === 'en' ? 'en-US' : locale, { numeric: 'auto' });