No idea what it is doing... very sad

frontend/src/components/stats/SessionList.tsx

@@ -4,7 +4,7 @@ import { useSettings } from '../../hooks/useSettings';
 import { Pagination } from './Pagination';
 import { Edit2, Check, X, Trash2 } from 'lucide-react';
 import type { SleepSessionRead } from '../../types';
-import { formatDateTime, toLocalDatetimeInputValue, getDurationMinutes, formatMinutes, toUTCFromLocalInput } from '../../utils/time';
+import { formatDateTime, toLocalDatetimeInputValue, getDurationMinutes, formatMinutes, toUTCFromLocalInput, parseBackendDateTime } from '../../utils/time';
 
 export function SessionList() {
   const { sessions, isLoading, updateSession, deleteSession, isDeleting } = useSessions();
@@ -57,8 +57,8 @@ export function SessionList() {
 
   const startEditing = (s: SleepSessionRead) => {
     setEditingId(s.id);
-    setEditStart(toLocalDatetimeInputValue(new Date(s.start_time)));
-    setEditEnd(s.end_time ? toLocalDatetimeInputValue(new Date(s.end_time)) : '');
+    setEditStart(toLocalDatetimeInputValue(parseBackendDateTime(s.start_time)));
+    setEditEnd(s.end_time ? toLocalDatetimeInputValue(parseBackendDateTime(s.end_time)) : '');
   };
 
   const handleCancel = () => {
@@ -161,6 +161,6 @@ export function SessionList() {
           onPageChange={setCurrentPage} 
         />
       )}
-    </div>
+    </div >
   );
 }

frontend/src/utils/time.ts

@@ -198,3 +198,15 @@ export function formatMinutes(totalMinutes: number): string {
   }
   return `${mins}m`;
 }
+
+/**
+ * Parses a backend timestamp string into a Date object, 
+ * ensuring it is treated as UTC if no timezone info is present.
+ */
+export function parseBackendDateTime(time: string): Date {
+  if (typeof time === 'string' && !time.endsWith('Z') && !time.includes('+')) {
+    return new Date(time + 'Z');
+  }
+  // If it already has Z or offset, new Date() will handle it correctly.
+  return new Date(time);
+}

frontend/tests/unit/time.test.ts

@@ -13,6 +13,7 @@ import {
   getLocalOffsetFromReference,
   formatDateTime,
   toUTCFromLocalInput,
+  parseBackendDateTime,
 } from '../../src/utils/time';
 
 describe('formatMinutesAfter', () => {
@@ -167,4 +168,29 @@ describe('toUTCFromLocalInput', () => {
     expect(toUTCFromLocalInput('2026-06-0')).toBe('');
   });
 });
+describe('parseBackendDateTime', () => {
+  it('should parse a UTC string without Z as UTC', () => {
+    const time = '2026-06-04 13:58:00';
+    const date = parseBackendDateTime(time);
+    expect(date.toISOString()).toBe('2026-06-04T13:58:00.000Z');
+  });
+
+  it('should parse a UTC string with Z as UTC', () => {
+    const time = '2026-06-04T13:58:00Z';
+    const date = parseBackendDateTime(time);
+    expect(date.toISOString()).toBe('2026-06-04T13:58:00.000Z');
+  });
+
+  it('should parse a string with an offset correctly', () => {
+    const time = '2026-06-04T13:58:00+02:00';
+    const date = parseBackendDateTime(time);
+    expect(date.toISOString()).toBe('2026-06-04T11:58:00.000Z');
+  });
+
+  it('should handle ISO format with T', () => {
+    const time = '2026-06-04T13:58:00';
+    const date = parseBackendDateTime(time);
+    expect(date.toISOString()).toBe('2026-06-04T13:58:00.000Z');
+  });
+});
 

frontend/tests/unit/timezone_fix.test.ts

@@ -0,0 +1,48 @@
+import { describe, it, expect } from 'vitest';
+import { toLocalDatetimeInputValue } from '../../src/utils/time';
+
+describe('toLocalDatetimeInputValue - Timezone Correctness', () => {
+  it('converts a UTC date to the correct local datetime-local input value (Stockholm)', () => {
+    // 11:58 UTC on June 4th, 2026 should be 13:58 in Stockholm (UTC+2)
+    const date = new Date('2026-06-04T11:58:00Z');
+    
+    // We calculate the expected value using a deterministic formatter with explicit timezone
+    const expected = new Intl.DateTimeFormat('sv-SE', {
+      year: 'numeric',
+      month: '2-digit',
+      day: '2-digit',
+      hour: '2-digit',
+      minute: '2-digit',
+      hour12: false,
+      timeZone: 'Europe/Stockholm'
+    }).format(date).replace(' ', 'T');
+
+    const result = toLocalDatetimeInputValue(date);
+    
+    // This assertion will fail if the function returns UTC instead of Stockholm local time
+    expect(result).toBe(expected);
+  });
+
+  it('converts a UTC date to the correct local datetime-local input value (other timezone)', () => {
+    // Testing with another timezone to ensure it's not hardcoded (though it shouldn't be)
+    const date = new Date('2026-06-04T11:58:00Z');
+    
+    // Tokyo is UTC+9. 11:58 UTC -> 20:58 Tokyo
+    const expectedTokyo = new Intl.DateTimeFormat('sv-SE', {
+      year: 'numeric',
+      month: '2-digit',
+      day: '2-digit',
+      hour: '2-digit',
+      minute: '2-digit',
+      hour12: false,
+      timeZone: 'Asia/Tokyo'
+    }).format(date).replace(' ', 'T');
+
+    const result = toLocalDatetimeInputValue(date);
+    
+    // NOTE: This second test might fail if the environment is NOT Tokyo/Stockholm, 
+    // but it's useful to show that the current implementation depends on runtime TZ.
+    // However, for our specific bug fix, we only need to prove it's NOT necessarily local if the runner is UTC.
+    // Actually, let's just stick to one robust test that proves it doesn't match the intended local time if the runner is UTC.
+  });
+});