test: add timezone boundary alignment coverage

Author: Sreeya-kumari

lib/calculate.test.ts

@@ -1319,6 +1319,30 @@ describe('calculateStreak — timezone awareness', () => {
     const result = calculateStreak(tzCalendar, 'UTC', nowUTC);
     expect(result.todayDate).toBe('2024-06-16');
   });
+
+  it('handles timezone boundary alignment between UTC, IST and JST', () => {
+    const calendar = {
+      totalContributions: 2,
+      weeks: [
+        {
+          contributionDays: [
+            { contributionCount: 1, date: '2024-06-14' },
+            { contributionCount: 1, date: '2024-06-15' },
+          ],
+        },
+      ],
+    };
+
+    const now = new Date('2024-06-14T20:00:00Z');
+
+    const utcResult = calculateStreak(calendar, 'UTC', now);
+    const istResult = calculateStreak(calendar, 'Asia/Kolkata', now);
+    const jstResult = calculateStreak(calendar, 'Asia/Tokyo', now);
+
+    expect(utcResult.todayDate).toBe('2024-06-14');
+    expect(istResult.todayDate).toBe('2024-06-15');
+    expect(jstResult.todayDate).toBe('2024-06-15');
+  });
 });
 
 describe('isStreakAlive', () => {