Align hour buckets to the site time zone (#7)

Fixes #6

Author: sinnercode228

packages/core/src/time.ts

@@ -66,9 +66,17 @@ export function startOfDay(ts: number, timeZone = 'UTC'): number {
 export function bucketStart(ts: number, granularity: Granularity, timeZone = 'UTC'): number {
   switch (granularity) {
     case 'minute':
+      // Every zone offset is a whole number of minutes, so UTC and local
+      // minute boundaries always coincide.
       return Math.floor(ts / MINUTE) * MINUTE;
-    case 'hour':
-      return Math.floor(ts / HOUR) * HOUR;
+    case 'hour': {
+      // Align to the local hour, like startOfDay does for the day: half-offset
+      // zones (e.g. Asia/Kolkata) start their hours at :30 past the UTC hour.
+      const offset = tzOffsetMinutes(ts, timeZone) * MINUTE;
+      const localHour = Math.floor((ts + offset) / HOUR) * HOUR;
+      const offsetAtHour = tzOffsetMinutes(localHour - offset, timeZone) * MINUTE;
+      return localHour - offsetAtHour;
+    }
     case 'day':
       return startOfDay(ts, timeZone);
   }

packages/core/test/time.test.ts

@@ -20,6 +20,27 @@ describe('time buckets', () => {
     expect(bucketStart(ts, 'hour')).toBe(at('2026-05-10T13:00:00Z'));
   });
 
+  it('aligns hour buckets to a half-offset zone', () => {
+    // 13:47 UTC is 19:17 in Kolkata (+5:30); the local hour starts at 19:00 IST = 13:30 UTC.
+    const ts = at('2026-05-10T13:47:31.500Z');
+    expect(bucketStart(ts, 'hour', 'Asia/Kolkata')).toBe(at('2026-05-10T13:30:00Z'));
+    // Minute buckets are unaffected: every zone offset is a whole number of minutes.
+    expect(bucketStart(ts, 'minute', 'Asia/Kolkata')).toBe(at('2026-05-10T13:47:00Z'));
+    expect(bucketsBetween(ts, ts + 2 * HOUR, 'hour', 'Asia/Kolkata')).toEqual([
+      at('2026-05-10T13:30:00Z'),
+      at('2026-05-10T14:30:00Z'),
+      at('2026-05-10T15:30:00Z'),
+    ]);
+  });
+
+  it('keeps hour buckets an hour apart across a whole-hour DST change', () => {
+    const tz = 'America/New_York';
+    // The fall-back hour (EDT -> EST) still steps by one real hour per local hour.
+    const b = bucketStart(at('2026-11-01T05:30:00Z'), 'hour', tz);
+    expect(b).toBe(at('2026-11-01T05:00:00Z'));
+    expect(nextBucket(b, 'hour', tz) - b).toBe(HOUR);
+  });
+
   it('computes local midnight for fixed-offset zones', () => {
     expect(tzOffsetMinutes(at('2026-05-10T12:00:00Z'), 'Europe/Moscow')).toBe(180);
     expect(startOfDay(at('2026-05-10T22:30:00Z'), 'Europe/Moscow')).toBe(