Add a Date & Time setting and fix inconsistent date formats app-wide

Different tables used different date formats: Servers & Tasks used a
fixed "YYYY-MM-DD HH:mm:ss" (24h), while Audit Log, DNS, Integrations
(Tailscale/Semaphore), and Users called plain .toLocaleString() with
no options, which renders using the browser's own locale — different
per browser/OS, and inconsistent with the other pages' fixed format.

- New Settings -> Date & Time page: pick date order (YYYY-MM-DD,
  DD/MM/YYYY, MM/DD/YYYY) and 12h vs 24h clock, with a live preview.
- utils/date.ts's formatDateTime() now reads these settings instead of
  being hardcoded to sv-SE/24h; the setting is fetched once at app
  startup (alongside /api/me) via a new non-secret GET
  /api/settings/display (any signed-in user, same rationale as the
  badge-color endpoints) and applied immediately on save too, without
  needing a page reload.
- Switched every remaining raw new Date(...).toLocaleString() call
  (Audit Log, DNS zone sync time, Tailscale/Semaphore last-seen,
  Users' last login) over to the shared formatter, so every table now
  renders dates identically.

Verified the formatter's date-order x 12h logic against all six
combinations plus the midnight/noon 12h edge cases, and the settings
endpoints end-to-end (defaults, partial updates, validation
rejection, audit logging) against the real dev server.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

web/src/App.tsx

@@ -14,8 +14,10 @@ import Integrations from "./pages/Integrations";
 import Settings from "./pages/Settings";
 import NotificationSettings from "./pages/settings/NotificationSettings";
 import BadgeSettings from "./pages/settings/BadgeSettings";
+import DateTimeSettings from "./pages/settings/DateTimeSettings";
 import CacheSettings from "./pages/settings/CacheSettings";
 import AppShell from "./layout/AppShell";
+import { setDateTimeSettings } from "./utils/date";
 
 const roleRank: Record<UserRole, number> = { viewer: 0, operator: 1, admin: 2 };
 
@@ -49,6 +51,7 @@ export default function App() {
         setUser(null);
       },
     );
+    api.settings.display().then((res) => setDateTimeSettings(res.display)).catch(() => {});
   }, []);
 
   if (user === undefined) {
@@ -96,6 +99,7 @@ export default function App() {
           <Route index element={<Navigate to="notifications" replace />} />
           <Route path="notifications" element={<NotificationSettings />} />
           <Route path="badges" element={<BadgeSettings />} />
+          <Route path="date-time" element={<DateTimeSettings />} />
           <Route path="cache" element={<CacheSettings />} />
         </Route>
       </Routes>

web/src/api/client.ts

@@ -119,6 +119,14 @@ export interface NotificationEvents {
   timezone: string;
 }
 
+export type DateFormat = "ymd" | "dmy" | "mdy";
+export type TimeFormat = "24h" | "12h";
+
+export interface DisplaySettings {
+  dateFormat: DateFormat;
+  timeFormat: TimeFormat;
+}
+
 export interface AppSettings {
   gotify: GotifySettings;
   ntfy: NtfySettings;
@@ -127,6 +135,7 @@ export interface AppSettings {
   notifications: NotificationEvents;
   providerColors: Record<string, string>;
   integrationColors: Record<string, string>;
+  display: DisplaySettings;
 }
 
 export type AppSettingsPatch = { [K in keyof AppSettings]?: Partial<AppSettings[K]> };
@@ -655,6 +664,7 @@ export const api = {
     get: () => request<{ settings: AppSettings }>("/api/settings"),
     providerColors: () => request<{ providerColors: Record<string, string> }>("/api/settings/provider-colors"),
     integrationColors: () => request<{ integrationColors: Record<string, string> }>("/api/settings/integration-colors"),
+    display: () => request<{ display: DisplaySettings }>("/api/settings/display"),
     update: (data: AppSettingsPatch) =>
       request<{ settings: AppSettings }>("/api/settings", { method: "PUT", body: JSON.stringify(data) }),
     testGotify: (data: { url: string; token: string; priority?: number }) =>

web/src/pages/AuditLog.tsx

@@ -1,5 +1,6 @@
 import { useEffect, useState } from "react";
 import { api, type AuditLogEntry } from "../api/client";
+import { formatDateTime } from "../utils/date";
 
 function targetLabel(e: AuditLogEntry): string {
   if (!e.targetType) return "—";
@@ -47,7 +48,7 @@ export default function AuditLog() {
             <tbody>
               {entries?.map((e) => (
                 <tr key={e.id}>
-                  <td>{new Date(e.createdAt).toLocaleString()}</td>
+                  <td>{formatDateTime(new Date(e.createdAt))}</td>
                   <td>{e.actorLabel ?? "—"}</td>
                   <td>
                     <span className="badge bg-blue-lt">{e.category}</span>

web/src/pages/Dns.tsx

@@ -9,6 +9,7 @@ import {
   type DnsZone,
 } from "../api/client";
 import DnsProviderForm from "../components/DnsProviderForm";
+import { formatDateTime } from "../utils/date";
 
 const PROVIDER_LABELS: Record<DnsProviderType, string> = {
   cloudflare: "Cloudflare",
@@ -315,7 +316,7 @@ export default function Dns({ user }: { user: CurrentUser }) {
                               <td>{z.name}</td>
                               <td>{z.recordCount}</td>
                               <td className="text-secondary">
-                                {z.syncedAt ? new Date(z.syncedAt).toLocaleString() : "never"}
+                                {z.syncedAt ? formatDateTime(new Date(z.syncedAt)) : "never"}
                               </td>
                             </tr>
                           ))}

web/src/pages/Integrations.tsx

@@ -17,6 +17,7 @@ import {
 } from "../api/client";
 import IntegrationForm from "../components/IntegrationForm";
 import IntegrationEditForm from "../components/IntegrationEditForm";
+import { formatDateTime } from "../utils/date";
 
 function proxmoxStatusBadge(status: string) {
   return status === "running" ? (
@@ -547,7 +548,7 @@ export default function Integrations({ user }: { user: CurrentUser }) {
                               )}
                               {!d.authorized && <span className="badge bg-yellow-lt text-yellow">Unauthorized</span>}
                             </td>
-                            <td className="text-secondary">{d.lastSeen ? new Date(d.lastSeen).toLocaleString() : "—"}</td>
+                            <td className="text-secondary">{d.lastSeen ? formatDateTime(new Date(d.lastSeen)) : "—"}</td>
                             {canEdit && (
                               <td>
                                 <div className="btn-list flex-nowrap">
@@ -775,7 +776,7 @@ export default function Integrations({ user }: { user: CurrentUser }) {
                             <td className="text-secondary">{t.projectName}</td>
                             <td className="text-secondary">
                               {t.lastTask
-                                ? new Date(t.lastTask.end ?? t.lastTask.start ?? t.lastTask.created ?? "").toLocaleString()
+                                ? formatDateTime(new Date(t.lastTask.end ?? t.lastTask.start ?? t.lastTask.created ?? ""))
                                 : "—"}
                             </td>
                             <td>{semaphoreStatusBadge(t)}</td>

web/src/pages/Settings.tsx

@@ -3,6 +3,7 @@ import { NavLink, Outlet } from "react-router-dom";
 const SUB_NAV = [
   { to: "/settings/notifications", label: "Notifications" },
   { to: "/settings/badges", label: "Badges" },
+  { to: "/settings/date-time", label: "Date & Time" },
   { to: "/settings/cache", label: "Cache" },
 ];
 

web/src/pages/Users.tsx

@@ -1,5 +1,6 @@
 import { useEffect, useState } from "react";
 import { api, type UserRecord, type UserRole } from "../api/client";
+import { formatDateTime } from "../utils/date";
 
 const ROLES: UserRole[] = ["viewer", "operator", "admin"];
 
@@ -64,7 +65,7 @@ export default function Users() {
                       ))}
                     </select>
                   </td>
-                  <td>{u.lastLoginAt ? new Date(u.lastLoginAt).toLocaleString() : "never"}</td>
+                  <td>{u.lastLoginAt ? formatDateTime(new Date(u.lastLoginAt)) : "never"}</td>
                 </tr>
               ))}
               {users?.length === 0 && (

web/src/pages/settings/DateTimeSettings.tsx

@@ -0,0 +1,110 @@
+import { useEffect, useState } from "react";
+import { api, type AppSettings, type DateFormat, type TimeFormat } from "../../api/client";
+import { setDateTimeSettings } from "../../utils/date";
+
+const DATE_FORMAT_OPTIONS: { value: DateFormat; label: string; example: string }[] = [
+  { value: "ymd", label: "Year-Month-Day", example: "2026-09-15" },
+  { value: "dmy", label: "Day/Month/Year", example: "15/09/2026" },
+  { value: "mdy", label: "Month/Day/Year", example: "09/15/2026" },
+];
+
+function previewTime(timeFormat: TimeFormat): string {
+  return timeFormat === "12h" ? "2:30:00 PM" : "14:30:00";
+}
+
+export default function DateTimeSettings() {
+  const [loading, setLoading] = useState(true);
+  const [loadError, setLoadError] = useState<string | null>(null);
+  const [dateFormat, setDateFormatState] = useState<DateFormat>("ymd");
+  const [timeFormat, setTimeFormatState] = useState<TimeFormat>("24h");
+  const [saving, setSaving] = useState(false);
+  const [saved, setSaved] = useState(false);
+  const [saveError, setSaveError] = useState<string | null>(null);
+
+  useEffect(() => {
+    api.settings
+      .get()
+      .then((res: { settings: AppSettings }) => {
+        setDateFormatState(res.settings.display.dateFormat);
+        setTimeFormatState(res.settings.display.timeFormat);
+      })
+      .catch((err) => setLoadError(err instanceof Error ? err.message : String(err)))
+      .finally(() => setLoading(false));
+  }, []);
+
+  async function handleSave() {
+    setSaving(true);
+    setSaveError(null);
+    setSaved(false);
+    try {
+      const res = await api.settings.update({ display: { dateFormat, timeFormat } });
+      setDateTimeSettings(res.settings.display);
+      setSaved(true);
+      setTimeout(() => setSaved(false), 3000);
+    } catch (err) {
+      setSaveError(err instanceof Error ? err.message : String(err));
+    } finally {
+      setSaving(false);
+    }
+  }
+
+  if (loading) return <div className="text-secondary">Loading…</div>;
+
+  const dateExample = DATE_FORMAT_OPTIONS.find((o) => o.value === dateFormat)?.example ?? "";
+
+  return (
+    <>
+      <div className="d-flex align-items-center justify-content-between mb-3">
+        <h3 className="mb-0">Date &amp; Time</h3>
+        <div className="d-flex align-items-center gap-2">
+          {saveError && <span className="text-danger small">{saveError}</span>}
+          {saved && <span className="text-success small">✓ Settings saved</span>}
+          <button className="btn btn-primary" onClick={handleSave} disabled={saving}>
+            {saving ? "Saving…" : "Save Settings"}
+          </button>
+        </div>
+      </div>
+
+      {loadError && <div className="alert alert-danger">{loadError}</div>}
+
+      <div className="card">
+        <div className="card-header">
+          <h3 className="card-title">Display format</h3>
+        </div>
+        <div className="card-body">
+          <p className="text-secondary">
+            Controls how dates and times are shown throughout the app — tables previously mixed formats depending on
+            the page; this makes them consistent everywhere.
+          </p>
+          <div className="row g-3">
+            <div className="col-md-6">
+              <label className="form-label">Date format</label>
+              <select className="form-select" value={dateFormat} onChange={(e) => setDateFormatState(e.target.value as DateFormat)}>
+                {DATE_FORMAT_OPTIONS.map((opt) => (
+                  <option key={opt.value} value={opt.value}>
+                    {opt.label} ({opt.example})
+                  </option>
+                ))}
+              </select>
+            </div>
+            <div className="col-md-6">
+              <label className="form-label">Clock</label>
+              <select className="form-select" value={timeFormat} onChange={(e) => setTimeFormatState(e.target.value as TimeFormat)}>
+                <option value="24h">24-hour (14:30:00)</option>
+                <option value="12h">12-hour (2:30:00 PM)</option>
+              </select>
+            </div>
+          </div>
+          <div className="mt-3">
+            <div className="text-secondary small">Preview</div>
+            <div>
+              <code>
+                {dateExample} {previewTime(timeFormat)}
+              </code>
+            </div>
+          </div>
+        </div>
+      </div>
+    </>
+  );
+}

web/src/utils/date.ts

@@ -1,12 +1,52 @@
-/** Formats a date as "YYYY-MM-DD HH:mm:ss" on a 24-hour clock. */
+import type { DateFormat, TimeFormat } from "../api/client";
+
+export interface DateTimeSettings {
+  dateFormat: DateFormat;
+  timeFormat: TimeFormat;
+}
+
+let current: DateTimeSettings = { dateFormat: "ymd", timeFormat: "24h" };
+
+/** Called once at app startup (and whenever the setting is changed) so every formatDateTime() call site picks it up without prop-drilling. */
+export function setDateTimeSettings(settings: DateTimeSettings): void {
+  current = settings;
+}
+
+function pad(n: number): string {
+  return String(n).padStart(2, "0");
+}
+
+/** Formats a date using the app-wide date/time display settings (defaults to "YYYY-MM-DD HH:mm:ss" on a 24-hour clock). */
 export function formatDateTime(date: Date): string {
-  return date.toLocaleString("sv-SE", {
-    year: "numeric",
-    month: "2-digit",
-    day: "2-digit",
-    hour: "2-digit",
-    minute: "2-digit",
-    second: "2-digit",
-    hour12: false,
-  });
+  const year = date.getFullYear();
+  const month = pad(date.getMonth() + 1);
+  const day = pad(date.getDate());
+
+  let datePart: string;
+  switch (current.dateFormat) {
+    case "dmy":
+      datePart = `${day}/${month}/${year}`;
+      break;
+    case "mdy":
+      datePart = `${month}/${day}/${year}`;
+      break;
+    case "ymd":
+    default:
+      datePart = `${year}-${month}-${day}`;
+      break;
+  }
+
+  const minutes = pad(date.getMinutes());
+  const seconds = pad(date.getSeconds());
+  let timePart: string;
+  if (current.timeFormat === "12h") {
+    const hour24 = date.getHours();
+    const period = hour24 >= 12 ? "PM" : "AM";
+    const hour12 = hour24 % 12 || 12;
+    timePart = `${hour12}:${minutes}:${seconds} ${period}`;
+  } else {
+    timePart = `${pad(date.getHours())}:${minutes}:${seconds}`;
+  }
+
+  return `${datePart} ${timePart}`;
 }