Fix timezones

frontend/tests/unit/components/stats/SleepTimetable.test.tsx

@@ -62,8 +62,8 @@ describe('SleepTimetable', () => {
     fireEvent(window, new Event('resize'));
 
     // Check for 3-day interval tabs
-    // In reversed order (25, 24, 23...), the first group is Fri - Wed
-    expect(screen.getByRole('button', { name: /Fri \d+ - Wed \d+/i })).toBeInTheDocument();
+    // In reversed order (25, 24, 23...), the first group is Wed - Fri
+    expect(screen.getByRole('button', { name: /Wed \d+ - Fri \d+/i })).toBeInTheDocument();
   });
 
   it('renders in small mode (single-day tabs and 1-column grid)', () => {

frontend/tests/unit/time.test.ts

@@ -11,6 +11,7 @@ import {
   timeToMinutes,
   adjustForTimezone,
   getLocalOffsetFromReference,
+  formatDateTime,
 } from '../../src/utils/time';
 
 describe('formatMinutesAfter', () => {
@@ -125,6 +126,21 @@ describe('adjustForTimezone', () => {
   });
 });
 
+describe('formatDateTime', () => {
+  it('treats string without Z as UTC', () => {
+    const dateStr = '2024-01-15T10:00:00';
+    const dateWithZ = '2024-01-15T10:00:00Z';
+    expect(formatDateTime(dateStr)).toBe(formatDateTime(dateWithZ));
+  });
+
+  it('formats a Date object correctly', () => {
+    const date = new Date('2024-01-15T10:00:00Z');
+    // The output depends on the local timezone, so we check for part of it
+    const result = formatDateTime(date);
+    expect(result).toMatch(/\d{4}-\d{2}-\d{2} \d{2}:\d{2}/);
+  });
+});
+
 describe('getLocalOffsetFromReference', () => {
   it('returns local offset from reference in minutes', () => {
     const utcMins = 600; // 10:00 UTC