From f29230c8a69e04c30f8f449c58009e79da9136a1 Mon Sep 17 00:00:00 2001
From: Jason Rasmussen <jason@rasm.me>
Date: Thu, 11 Sep 2025 15:36:16 -0400
Subject: [PATCH] fix(web): handle buckets before year 1000 (#21832)

---
 web/src/lib/utils/timeline-util.spec.ts | 12 +++++++++++-
 web/src/lib/utils/timeline-util.ts      |  7 +++++--
 2 files changed, 16 insertions(+), 3 deletions(-)

diff --git a/web/src/lib/utils/timeline-util.spec.ts b/web/src/lib/utils/timeline-util.spec.ts
index c77aefc0b4..bc05b78ad4 100644
--- a/web/src/lib/utils/timeline-util.spec.ts
+++ b/web/src/lib/utils/timeline-util.spec.ts
@@ -1,6 +1,6 @@
 import { locale } from '$lib/stores/preferences.store';
 import { parseUtcDate } from '$lib/utils/date-time';
-import { formatGroupTitle } from '$lib/utils/timeline-util';
+import { formatGroupTitle, toISOYearMonthUTC } from '$lib/utils/timeline-util';
 import { DateTime } from 'luxon';
 
 describe('formatGroupTitle', () => {
@@ -77,3 +77,13 @@ describe('formatGroupTitle', () => {
     expect(formatGroupTitle(date)).toBe('Invalid DateTime');
   });
 });
+
+describe('toISOYearMonthUTC', () => {
+  it('should prefix year with 0s', () => {
+    expect(toISOYearMonthUTC({ year: 28, month: 1 })).toBe('0028-01-01T00:00:00.000Z');
+  });
+
+  it('should prefix month with 0s', () => {
+    expect(toISOYearMonthUTC({ year: 2025, month: 1 })).toBe('2025-01-01T00:00:00.000Z');
+  });
+});
diff --git a/web/src/lib/utils/timeline-util.ts b/web/src/lib/utils/timeline-util.ts
index 6a0f12c20e..9cf4428da6 100644
--- a/web/src/lib/utils/timeline-util.ts
+++ b/web/src/lib/utils/timeline-util.ts
@@ -94,8 +94,11 @@ export const fromTimelinePlainYearMonth = (timelineYearMonth: TimelineYearMonth)
     { zone: 'local', locale: get(locale) },
   ) as DateTime<true>;
 
-export const toISOYearMonthUTC = ({ year, month }: TimelineYearMonth): string =>
-  `${year}-${month.toString().padStart(2, '0')}-01T00:00:00.000Z`;
+export const toISOYearMonthUTC = ({ year, month }: TimelineYearMonth): string => {
+  const yearFull = `${year}`.padStart(4, '0');
+  const monthFull = `${month}`.padStart(2, '0');
+  return `${yearFull}-${monthFull}-01T00:00:00.000Z`;
+};
 
 export function formatMonthGroupTitle(_date: DateTime): string {
   if (!_date.isValid) {