test(utils): check boundary robustness of time normalization (Variati… (JhaSourav07#1645)

…on 3)

## Description
test(utils): check boundary robustness of time normalization (Variation
3)

Fixes JhaSourav07#1569

## Pillar

- [ ] 🎨 Pillar 1 — New Theme Design
- [ ] 📐 Pillar 2 — Geometric SVG Improvement
- [ ] 🕐 Pillar 3 — Timezone Logic Optimization
- [x] 🛠️ Other (Bug fix, refactoring, docs)

## Visual Preview

## Checklist before requesting a review:

- [x] I have read the `CONTRIBUTING.md` file.
- [x] I have tested these changes locally
(`localhost:3000/api/streak?user=YOUR_USERNAME`).
- [x] I have run `npm run format` and `npm run lint` locally and
resolved all errors (CI will fail otherwise).
- [x] My commits follow the Conventional Commits format (e.g.,
`feat(themes): ...`, `fix(calculate): ...`).
- [x] I have updated `README.md` if I added a new theme or URL
parameter.
- [x] I have started the repo.
- [x] I have made sure that i have only one commit to merge in this PR.
- [x] The SVG output matches the CommitPulse "premium quality" aesthetic
standard (no raw elements, smooth animations, correct fonts).
- [x] (Recommended) I joined the CommitPulse Discord community for
contributor discussions, mentorship, and faster PR support.

Author: JhaSourav07

components/ReturnToTop.test.tsx

@@ -1,5 +1,5 @@
 import { render } from '@testing-library/react';
-import { describe, it, expect, vi, beforeEach, afterEach } from 'vitest';
+import { describe, it, expect, vi, beforeEach } from 'vitest';
 import ReturnToTop from './ReturnToTop';
 import type React from 'react';
 import { screen, fireEvent } from '@testing-library/react';

utils/time.test.ts

@@ -151,6 +151,59 @@ describe('getSecondsUntilMidnightInTimezone', () => {
 
     expect(getSecondsUntilMidnightInTimezone('Etc/GMT-14')).toBe(10 * 3600);
   });
+
+  it('should calculate seconds until UTC midnight correctly at a calendar boundary', () => {
+    // Arrange: Set time to Dec 31, 2023, 23:59:50 UTC (10 seconds before New Year)
+    const boundaryTime = new Date(Date.UTC(2023, 11, 31, 23, 59, 50));
+    vi.setSystemTime(boundaryTime);
+
+    // Act
+    const seconds = getSecondsUntilUTCMidnight();
+
+    // Assert: Exactly 10 seconds should remain
+    expect(seconds).toBe(10);
+  });
+
+  it('should handle extreme positive timezone offset boundary (+14:00)', () => {
+    // Arrange: Pacific/Kiritimati is UTC+14.
+    // When UTC is Dec 31, 09:59:50, Kiritimati is Dec 31, 23:59:50 (10 seconds to midnight)
+    const boundaryTime = new Date(Date.UTC(2023, 11, 31, 9, 59, 50));
+    vi.setSystemTime(boundaryTime);
+
+    // Act
+    const seconds = getSecondsUntilMidnightInTimezone('Pacific/Kiritimati');
+
+    // Assert
+    expect(seconds).toBe(10);
+  });
+
+  it('should handle extreme negative timezone offset boundary (-11:00)', () => {
+    // Arrange: Pacific/Midway is UTC-11.
+    // When UTC is Jan 1, 10:59:50, Midway is Dec 31, 23:59:50 (10 seconds to midnight)
+    const boundaryTime = new Date(Date.UTC(2024, 0, 1, 10, 59, 50));
+    vi.setSystemTime(boundaryTime);
+
+    // Act
+    const seconds = getSecondsUntilMidnightInTimezone('Pacific/Midway');
+
+    // Assert
+    expect(seconds).toBe(10);
+  });
+
+  it('should handle exact midnight boundary gracefully (0 seconds remaining)', () => {
+    // Arrange: Exactly midnight in UTC
+    const midnightTime = new Date(Date.UTC(2024, 0, 1, 0, 0, 0));
+    vi.setSystemTime(midnightTime);
+
+    // Act
+    const secondsUTC = getSecondsUntilUTCMidnight();
+    const secondsLondon = getSecondsUntilMidnightInTimezone('Europe/London');
+
+    // Assert: Both should recognize it's 24 hours (86400 seconds) until the NEXT midnight
+    // Since the logic does `86400 - (hour * 3600 + ...)`, at 00:00:00 it returns 86400.
+    expect(secondsUTC).toBe(86400);
+    expect(secondsLondon).toBe(86400);
+  });
 });
 
 describe('getSecondsUntilUTCMidnight — sliding window boundary robustness', () => {