Phase 0.3: migrate date formatting to date-fns

frontend/package-lock.json

@@ -23,6 +23,7 @@
         "@tiptap/starter-kit": "^3.28.0",
         "axios": "^1.7.7",
         "clsx": "^2.1.1",
+        "date-fns": "^4.4.0",
         "i18next": "^23.14.0",
         "i18next-browser-languagedetector": "^8.0.0",
         "lucide-react": "^1.25.0",
@@ -2747,6 +2748,15 @@
         "node": ">=18"
       }
     },
+    "node_modules/date-fns": {
+      "version": "4.4.0",
+      "resolved": "https://registry.npmjs.org/date-fns/-/date-fns-4.4.0.tgz",
+      "integrity": "sha512-+1UMbeh68lH1SegH83CGWwpb6OHHbpSgr3+s5Eww5M4CAgswBpoWS0AjTOfEJ33HiYKz1hdj/KTFprzXHmq/6w==",
+      "funding": {
+        "type": "github",
+        "url": "https://github.com/sponsors/kossnocorp"
+      }
+    },
     "node_modules/debug": {
       "version": "4.4.3",
       "resolved": "https://registry.npmjs.org/debug/-/debug-4.4.3.tgz",

frontend/package.json

@@ -27,6 +27,7 @@
     "@tiptap/starter-kit": "^3.28.0",
     "axios": "^1.7.7",
     "clsx": "^2.1.1",
+    "date-fns": "^4.4.0",
     "i18next": "^23.14.0",
     "i18next-browser-languagedetector": "^8.0.0",
     "lucide-react": "^1.25.0",

frontend/src/api/mail.ts

@@ -438,13 +438,14 @@ export function decodeMimeHeader(value: string | undefined | null): string {
  * Variables: {{user.name}}, {{user.first_name}}, {{user.last_name}},
  * {{user.email}}, {{user.role}}, {{tenant.name}}, {{date}}
  */
+import { formatDateShort } from '@/utils/date';
+
 export function replaceSignatureVariables(
   html: string,
   user: { name?: string; email?: string; role?: string; first_name?: string; last_name?: string },
   tenant?: { name?: string },
 ): string {
-  const now = new Date();
-  const dateStr = now.toLocaleDateString([], { year: 'numeric', month: '2-digit', day: '2-digit' });
+  const dateStr = formatDateShort(new Date());
   const firstName = user.first_name || (user.name ? user.name.split(' ')[0] : '');
   const lastName = user.last_name || (user.name ? user.name.split(' ').slice(1).join(' ') : '');
 

frontend/src/components/calendar/AppointmentModal.tsx

@@ -21,6 +21,7 @@ import {
   type EntryPriority,
   type EntrySubtype,
 } from '@/api/calendar';
+import { formatDateTimeInput } from '@/utils/date';
 
 export interface AppointmentModalProps {
   open: boolean;
@@ -41,9 +42,7 @@ export interface AppointmentModalProps {
 
 function toDateTimeLocalValue(d: Date | null | undefined): string {
   if (!d) return '';
-  // datetime-local needs YYYY-MM-DDTHH:mm in local time
-  const pad = (n: number) => String(n).padStart(2, '0');
-  return `${d.getFullYear()}-${pad(d.getMonth() + 1)}-${pad(d.getDate())}T${pad(d.getHours())}:${pad(d.getMinutes())}`;
+  return formatDateTimeInput(d);
 }
 
 function fromDateTimeLocalValue(v: string): Date | null {

frontend/src/components/calendar/CalendarDetail.tsx

@@ -21,21 +21,11 @@ import {
 import { Button } from '@/components/ui/Button';
 import { Input } from '@/components/ui/Input';
 import { Calendar as CalendarIcon, Pencil, Trash2, X } from 'lucide-react';
+import { formatDateTime } from '@/utils/date';
 
 function formatDate(dateStr: string | null | undefined): string {
   if (!dateStr) return '-';
-  try {
-    const d = new Date(dateStr);
-    return d.toLocaleDateString(undefined, {
-      year: 'numeric',
-      month: 'short',
-      day: 'numeric',
-      hour: '2-digit',
-      minute: '2-digit',
-    });
-  } catch {
-    return dateStr;
-  }
+  return formatDateTime(dateStr) || '-';
 }
 
 function priorityClass(p: string): string {

frontend/src/components/calendar/KanbanBoard.tsx

@@ -7,6 +7,7 @@ import React, { useState } from 'react';
 import { useTranslation } from 'react-i18next';
 import clsx from 'clsx';
 import type { CalendarEntry } from '@/api/calendar';
+import { formatDateShort } from '@/utils/date';
 
 export type KanbanStatus = 'open' | 'in_progress' | 'done' | 'cancelled';
 
@@ -38,8 +39,7 @@ function priorityBadgeClass(p: string): string {
 
 function fmtDue(entry: CalendarEntry): string | null {
   if (!entry.due_date) return null;
-  const d = new Date(entry.due_date);
-  return d.toLocaleDateString('de-DE', { day: '2-digit', month: '2-digit', year: 'numeric' });
+  return formatDateShort(entry.due_date) || null;
 }
 
 export function KanbanBoard({ board, loading, onMove, onSelect }: KanbanBoardProps) {

frontend/src/components/calendar/TaskDetailPanel.tsx

@@ -15,6 +15,7 @@ import {
   type CalendarEntry,
   type Subtask,
 } from '@/api/calendar';
+import { formatDateShort } from '@/utils/date';
 import { Button } from '@/components/ui/Button';
 import { Input } from '@/components/ui/Input';
 
@@ -129,7 +130,7 @@ export function TaskDetailPanel({ entry, onClose, onChanged }: TaskDetailPanelPr
           </span>
           {entry.due_date && (
             <span className="px-2 py-0.5 rounded bg-primary-50 text-primary-700">
-              {new Date(entry.due_date).toLocaleDateString('de-DE')}
+              {formatDateShort(entry.due_date)}
             </span>
           )}
         </div>

frontend/src/components/dms/FileDetails.tsx

@@ -9,15 +9,11 @@ import { useTranslation } from 'react-i18next';
 import type { DmsFile } from '@/api/dms';
 import { getFileIcon, formatFileSize } from './FileExplorer';
 import { Download, Eye, FileText, Share2, Trash2, X } from 'lucide-react';
+import { formatDateTime } from '@/utils/date';
 
 function formatDate(dateStr: string | null | undefined): string {
   if (!dateStr) return '-';
-  try {
-    const d = new Date(dateStr);
-    return d.toLocaleDateString(undefined, { year: 'numeric', month: 'short', day: 'numeric', hour: '2-digit', minute: '2-digit' });
-  } catch {
-    return dateStr;
-  }
+  return formatDateTime(dateStr) || '-';
 }
 
 function getFileSize(file: DmsFile): number {

frontend/src/components/dms/FileExplorer.tsx

@@ -38,6 +38,8 @@ export function formatFileSize(bytes: number): string {
   if (bytes < 1024 * 1024 * 1024) return `${(bytes / (1024 * 1024)).toFixed(1)} MB`;
   return `${(bytes / (1024 * 1024 * 1024)).toFixed(1)} GB`;
 }
+import { formatDateShort } from '@/utils/date';
+
 
 function getFileSize(file: DmsFile): number {
   return file.size_bytes ?? file.size ?? 0;
@@ -45,12 +47,7 @@ function getFileSize(file: DmsFile): number {
 
 function formatDate(dateStr: string | null | undefined): string {
   if (!dateStr) return '';
-  try {
-    const d = new Date(dateStr);
-    return d.toLocaleDateString(undefined, { year: 'numeric', month: 'short', day: 'numeric' });
-  } catch {
-    return dateStr;
-  }
+  return formatDateShort(dateStr) || '';
 }
 
 export type ViewMode = 'list' | 'table' | 'icons-sm' | 'icons-md' | 'icons-lg';

frontend/src/components/dms/FilePreviewModal.tsx

@@ -8,6 +8,7 @@ import React, { useEffect, useState } from 'react';
 import { useTranslation } from 'react-i18next';
 import { Modal } from '@/components/ui/Modal';
 import { Button } from '@/components/ui/Button';
+import { formatDateShort } from '@/utils/date';
 import { Badge } from '@/components/ui/Badge';
 import { getFilePreviewUrl, type DmsFile } from '@/api/dms';
 import { FileText } from 'lucide-react';
@@ -56,7 +57,7 @@ export function FilePreviewModal({ open, file, onClose }: FilePreviewModalProps)
           <span className="text-sm text-secondary-500">{formatFileSize(file.size_bytes ?? file.size ?? 0)}</span>
           {file.created_at && (
             <span className="text-sm text-secondary-500">
-              {t('dms.fileModified')}: {new Date(file.created_at).toLocaleDateString()}
+              {t('dms.fileModified')}: {formatDateShort(file.created_at)}
             </span>
           )}
         </div>

frontend/src/components/dms/ShareDialog.tsx

@@ -8,6 +8,7 @@ import { useTranslation } from 'react-i18next';
 import { Modal } from '@/components/ui/Modal';
 import { Button } from '@/components/ui/Button';
 import { Input } from '@/components/ui/Input';
+import { formatDateShort } from '@/utils/date';
 import { Select } from '@/components/ui/Select';
 import { Badge } from '@/components/ui/Badge';
 import { EmptyState } from '@/components/ui/EmptyState';
@@ -259,7 +260,7 @@ export function ShareDialog({ open, file, onClose, onShared }: ShareDialogProps)
                     )}
                     {link.expires_at && (
                       <Badge variant="info">
-                        {t('permissions.expiresAt')}: {new Date(link.expires_at).toLocaleDateString()}
+                        {t('permissions.expiresAt')}: {formatDateShort(link.expires_at)}
                       </Badge>
                     )}
                   </div>

frontend/src/components/layout/MessageSidebar.tsx

@@ -1,4 +1,5 @@
 import React, { useState, useEffect, useRef, useCallback } from 'react';
+import { formatTime } from '@/utils/date';
 import { ResizablePanel } from '@/components/ui/ResizablePanel';
 import { useUIStore } from '@/store/uiStore';
 import { useCommStore } from '@/store/commStore';
@@ -241,7 +242,7 @@ function MessageFeed({
               )}
               {msg.created_at && (
                 <span className="text-[10px] opacity-50 mt-0.5 block">
-                  {new Date(msg.created_at).toLocaleTimeString('de-DE', { hour: '2-digit', minute: '2-digit' })}
+                  {formatTime(msg.created_at)}
                 </span>
               )}
             </div>

frontend/src/components/mail/MailDetail.tsx

@@ -11,6 +11,7 @@ import { decodeMimeHeader } from '@/api/mail';
 import { Button } from '@/components/ui/Button';
 import { EmptyState } from '@/components/ui/EmptyState';
 import { FileText, Loader2 } from 'lucide-react';
+import { formatDateTime } from '@/utils/date';
 
 export interface MailDetailProps {
   mail: Mail | null; loading: boolean;
@@ -26,14 +27,7 @@ export interface MailDetailProps {
 }
 
 function formatFullDate(dateStr: string): string {
-  const date = new Date(dateStr);
-  return date.toLocaleString([], {
-    year: 'numeric',
-    month: 'short',
-    day: 'numeric',
-    hour: '2-digit',
-    minute: '2-digit',
-  });
+  return formatDateTime(dateStr) || dateStr;
 }
 
 function formatBytes(bytes: number): string {

frontend/src/components/mail/MailList.tsx

@@ -11,6 +11,7 @@ import { decodeMimeHeader } from '@/api/mail';
 import { EmptyState } from '@/components/ui/EmptyState';
 import { Pagination } from '@/components/ui/Pagination';
 import { ChevronUp, Loader2, Paperclip, Star } from 'lucide-react';
+import { formatSmartDate } from '@/utils/date';
 
 export interface MailListProps {
   mails: Mail[];
@@ -30,12 +31,7 @@ export interface MailListProps {
 }
 
 function formatDate(dateStr: string): string {
-  const date = new Date(dateStr);
- const now = new Date();
-  if (date.toDateString() === now.toDateString()) {
-    return date.toLocaleTimeString([], { hour: '2-digit', minute: '2-digit' });
-  }
-  return date.toLocaleDateString([], { month: 'short', day: 'numeric' });
+  return formatSmartDate(dateStr) || dateStr;
 }
 
 export function MailList({

frontend/src/pages/AuditLog.tsx

@@ -8,6 +8,7 @@ import { Input } from '@/components/ui/Input';
 import { Button } from '@/components/ui/Button';
 import { EmptyState } from '@/components/ui/EmptyState';
 import { Badge } from '@/components/ui/Badge';
+import { formatDateTime } from '@/utils/date';
 
 export function AuditLogPage() {
   const { t } = useTranslation();
@@ -41,8 +42,7 @@ export function AuditLogPage() {
         cell: (info) => {
           const val = info.getValue();
           if (!val) return '—';
-          const date = new Date(val);
-          return date.toLocaleString('de-DE');
+          return formatDateTime(val) || '—';
         },
       },
       {

frontend/src/pages/Calendar.tsx

@@ -18,6 +18,7 @@ import { MonthView } from '@/components/calendar/MonthView';
 import { WeekView } from '@/components/calendar/WeekView';
 import { DayView } from '@/components/calendar/DayView';
 import { RangeView } from '@/components/calendar/RangeView';
+import { formatDateInput } from '@/utils/date';
 import { CalendarDetail } from '@/components/calendar/CalendarDetail';
 import { AppointmentModal } from '@/components/calendar/AppointmentModal';
 import { IcsControls } from '@/components/calendar/IcsControls';
@@ -488,7 +489,7 @@ export function CalendarPage() {
         <label className="text-xs text-secondary-600">{t('calendar.range.start')}</label>
         <input
           type="date"
-          value={rangeStart ? rangeStart.toISOString().slice(0, 10) : ''}
+          value={rangeStart ? formatDateInput(rangeStart) : ''}
           onChange={(e) => setRangeStart(e.target.value ? new Date(e.target.value) : null)}
           className="text-sm rounded-md border border-secondary-300 px-2 py-1"
           data-testid="calendar-range-start"
@@ -496,7 +497,7 @@ export function CalendarPage() {
         <label className="text-xs text-secondary-600">{t('calendar.range.end')}</label>
         <input
           type="date"
-          value={rangeEnd ? rangeEnd.toISOString().slice(0, 10) : ''}
+          value={rangeEnd ? formatDateInput(rangeEnd) : ''}
           onChange={(e) => setRangeEnd(e.target.value ? new Date(e.target.value) : null)}
           className="text-sm rounded-md border border-secondary-300 px-2 py-1"
           data-testid="calendar-range-end"

frontend/src/pages/Dashboard.tsx

@@ -3,6 +3,7 @@ import { useTranslation } from 'react-i18next';
 import { StatCard } from '@/components/shared/StatCard';
 import { ActivityFeed, ActivityItem } from '@/components/shared/ActivityFeed';
 import { useUnifiedContacts, useAuditLog } from '@/api/hooks';
+import { formatDateTime } from '@/utils/date';
 
 export function DashboardPage() {
   const { t } = useTranslation();
@@ -36,7 +37,7 @@ export function DashboardPage() {
         id: `${entry.timestamp}-${entry.user}-${entry.action}`,
         user: entry.user || 'System',
         action: entry.action || '',
-        time: entry.timestamp ? new Date(entry.timestamp).toLocaleString('de-DE') : '',
+        time: entry.timestamp ? (formatDateTime(entry.timestamp) || '') : '',
         avatarUrl: null,
       }));
 

frontend/src/pages/DmsTrash.tsx

@@ -8,6 +8,7 @@ import { useTranslation } from 'react-i18next';
 import { Link } from 'react-router-dom';
 import { Button } from '@/components/ui/Button';
 import { Card } from '@/components/ui/Card';
+import { formatDateShort } from '@/utils/date';
 import { Table, TableColumn } from '@/components/ui/Table';
 import { EmptyState } from '@/components/ui/EmptyState';
 import { Skeleton } from '@/components/ui/Skeleton';
@@ -79,7 +80,7 @@ export function DmsTrashPage() {
       sortable: true,
       accessor: (file) => file.deleted_at || '',
       render: (file) => file.deleted_at
-        ? new Date(file.deleted_at).toLocaleDateString()
+        ? (formatDateShort(file.deleted_at) || '—')
         : '—',
     },
     {

frontend/src/utils/date.ts

@@ -0,0 +1,86 @@
+import { format, formatDistanceToNow, parseISO, isValid, isToday } from 'date-fns';
+import { de } from 'date-fns/locale';
+
+/**
+ * Parse a date string or Date object into a valid Date, or null if invalid/empty.
+ */
+function toDate(value: string | Date | null | undefined): Date | null {
+  if (value === null || value === undefined || value === '') return null;
+  if (value instanceof Date) return isValid(value) ? value : null;
+  const parsed = parseISO(value);
+  return isValid(parsed) ? parsed : null;
+}
+
+/**
+ * Format a date with a custom date-fns pattern and German locale.
+ * Returns empty string for null/undefined/invalid dates.
+ */
+export function formatDate(date: string | Date | null | undefined, pattern: string): string {
+  const d = toDate(date);
+  if (!d) return '';
+  return format(d, pattern, { locale: de });
+}
+
+/**
+ * Standard date + time format: dd.MM.yyyy HH:mm
+ */
+export function formatDateTime(date: string | Date | null | undefined): string {
+  return formatDate(date, 'dd.MM.yyyy HH:mm');
+}
+
+/**
+ * Short date format: dd.MM.yyyy
+ */
+export function formatDateShort(date: string | Date | null | undefined): string {
+  return formatDate(date, 'dd.MM.yyyy');
+}
+
+/**
+ * Time only format: HH:mm
+ */
+export function formatTime(date: string | Date | null | undefined): string {
+  return formatDate(date, 'HH:mm');
+}
+
+/**
+ * Relative time format: "vor 5 Minuten", "in 2 Stunden"
+ */
+export function formatRelative(date: string | Date | null | undefined): string {
+  const d = toDate(date);
+  if (!d) return '';
+  return formatDistanceToNow(d, { locale: de, addSuffix: true });
+}
+
+/**
+ * Month + day format: "15. Juli"
+ */
+export function formatMonthDay(date: string | Date | null | undefined): string {
+  return formatDate(date, 'd. MMMM');
+}
+
+/**
+ * Smart date: shows time (HH:mm) if today, otherwise short date (d. MMM).
+ * Used in mail list and similar compact views.
+ */
+export function formatSmartDate(date: string | Date | null | undefined): string {
+  const d = toDate(date);
+  if (!d) return '';
+  if (isToday(d)) {
+    return format(d, 'HH:mm', { locale: de });
+  }
+  return format(d, 'd. MMM', { locale: de });
+}
+
+/**
+ * Date input format for <input type="date">: yyyy-MM-dd
+ */
+export function formatDateInput(date: string | Date | null | undefined): string {
+  return formatDate(date, 'yyyy-MM-dd');
+}
+
+/**
+ * DateTime input format for <input type="datetime-local">: yyyy-MM-ddTHH:mm
+ */
+export function formatDateTimeInput(date: string | Date | null | undefined): string {
+  return formatDate(date, "yyyy-MM-dd'T'HH:mm");
+}