feat: timezone setting — display timestamps in selected local timezone

Add a Display section to the settings modal with a timezone dropdown.
Selection is persisted to localStorage and applied to all timestamps via
fmtDate (date-only) and fmtDateFull (date + time + TZ abbreviation, e.g.
"Mar 28, 2026, 2:48 PM EDT"). Changing the timezone live-re-renders the
current page. Defaults to UTC.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

css/app.css

@@ -665,6 +665,9 @@ select:focus { border-color: var(--accent); }
   margin-bottom: 8px;
 }
 .settings-desc { font-size: 12px; color: var(--text2); margin: 0 0 14px; line-height: 1.6; }
+.settings-row { display: flex; align-items: center; gap: 12px; }
+.settings-label { font-size: 13px; color: var(--text2); white-space: nowrap; min-width: 80px; }
+.settings-select { flex: 1; }
 .import-row { display: flex; gap: 10px; align-items: center; }
 .import-file-input { flex: 1; }
 

index.html

@@ -180,6 +180,13 @@
       <button class="modal-close" onclick="closeSettingsModal()">&#x2715;</button>
     </div>
     <div class="modal-body">
+      <div class="settings-section">
+        <div class="settings-section-title">Display</div>
+        <div class="settings-row">
+          <label class="settings-label" for="tz-select">Timezone</label>
+          <select id="tz-select" class="form-input settings-select"></select>
+        </div>
+      </div>
       <div class="settings-section">
         <div class="settings-section-title">Export</div>
         <p class="settings-desc">Download all instance data as a JSON backup file.</p>

js/ui.js

@@ -3,6 +3,34 @@ let editingVmid = null;
 let currentVmid = null;
 let toastTimer  = null;
 
+// ── Timezone ──────────────────────────────────────────────────────────────────
+
+const TIMEZONES = [
+  { label: 'UTC',                      tz: 'UTC' },
+  { label: 'Hawaii (HST)',             tz: 'Pacific/Honolulu' },
+  { label: 'Alaska (AKT)',             tz: 'America/Anchorage' },
+  { label: 'Pacific (PT)',             tz: 'America/Los_Angeles' },
+  { label: 'Mountain (MT)',            tz: 'America/Denver' },
+  { label: 'Central (CT)',             tz: 'America/Chicago' },
+  { label: 'Eastern (ET)',             tz: 'America/New_York' },
+  { label: 'Atlantic (AT)',            tz: 'America/Halifax' },
+  { label: 'London (GMT/BST)',         tz: 'Europe/London' },
+  { label: 'Paris / Berlin (CET)',     tz: 'Europe/Paris' },
+  { label: 'Helsinki (EET)',           tz: 'Europe/Helsinki' },
+  { label: 'Istanbul (TRT)',           tz: 'Europe/Istanbul' },
+  { label: 'Dubai (GST)',              tz: 'Asia/Dubai' },
+  { label: 'India (IST)',              tz: 'Asia/Kolkata' },
+  { label: 'Singapore (SGT)',          tz: 'Asia/Singapore' },
+  { label: 'China (CST)',              tz: 'Asia/Shanghai' },
+  { label: 'Japan / Korea (JST/KST)', tz: 'Asia/Tokyo' },
+  { label: 'Sydney (AEST)',            tz: 'Australia/Sydney' },
+  { label: 'Auckland (NZST)',          tz: 'Pacific/Auckland' },
+];
+
+function getTimezone() {
+  return localStorage.getItem('catalyst_tz') || 'UTC';
+}
+
 // ── Helpers ───────────────────────────────────────────────────────────────────
 
 function esc(str) {
@@ -14,14 +42,14 @@ function esc(str) {
 function fmtDate(d) {
   if (!d) return '—';
   try {
-    return new Date(d).toLocaleDateString('en-US', { year: 'numeric', month: 'short', day: 'numeric' });
+    return new Date(d).toLocaleDateString('en-US', { year: 'numeric', month: 'short', day: 'numeric', timeZone: getTimezone() });
   } catch (e) { return d; }
 }
 
 function fmtDateFull(d) {
   if (!d) return '—';
   try {
-    return new Date(d).toLocaleString('en-US', { year: 'numeric', month: 'short', day: 'numeric', hour: '2-digit', minute: '2-digit' });
+    return new Date(d).toLocaleString('en-US', { year: 'numeric', month: 'short', day: 'numeric', hour: '2-digit', minute: '2-digit', timeZone: getTimezone(), timeZoneName: 'short' });
   } catch (e) { return d; }
 }
 
@@ -289,6 +317,16 @@ function showToast(msg, type = 'success') {
 // ── Settings Modal ────────────────────────────────────────────────────────────
 
 function openSettingsModal() {
+  const sel = document.getElementById('tz-select');
+  if (!sel.options.length) {
+    for (const { label, tz } of TIMEZONES) {
+      const opt = document.createElement('option');
+      opt.value = tz;
+      opt.textContent = label;
+      sel.appendChild(opt);
+    }
+  }
+  sel.value = getTimezone();
   document.getElementById('settings-modal').classList.add('open');
 }
 
@@ -353,3 +391,10 @@ document.getElementById('confirm-overlay').addEventListener('click', e => {
 document.getElementById('settings-modal').addEventListener('click', e => {
   if (e.target === document.getElementById('settings-modal')) closeSettingsModal();
 });
+
+document.getElementById('tz-select').addEventListener('change', e => {
+  localStorage.setItem('catalyst_tz', e.target.value);
+  const m = window.location.pathname.match(/^\/instance\/(\d+)/);
+  if (m) renderDetailPage(parseInt(m[1], 10));
+  else renderDashboard();
+});

tests/helpers.test.js

@@ -88,24 +88,36 @@ describe('fmtDate', () => {
 
 // ── fmtDateFull() ─────────────────────────────────────────────────────────────
 
-function fmtDateFull(d) {
+function fmtDateFull(d, tz = 'UTC') {
   if (!d) return '—'
   try {
     return new Date(d).toLocaleString('en-US', {
       year: 'numeric', month: 'short', day: 'numeric',
       hour: '2-digit', minute: '2-digit',
+      timeZone: tz, timeZoneName: 'short',
     })
   } catch (e) { return d }
 }
 
 describe('fmtDateFull', () => {
   it('includes date and time components', () => {
-    const result = fmtDateFull('2024-03-15T14:30:00')
+    const result = fmtDateFull('2024-03-15T14:30:00Z')
     expect(result).toMatch(/Mar/)
     expect(result).toMatch(/2024/)
     expect(result).toMatch(/\d{1,2}:\d{2}/)
   })
 
+  it('includes the timezone abbreviation', () => {
+    expect(fmtDateFull('2024-03-15T14:30:00Z', 'UTC')).toMatch(/UTC/)
+  })
+
+  it('converts to the given timezone', () => {
+    // 2024-03-15 18:30 UTC = 2024-03-15 14:30 EDT (UTC-4 in March)
+    const result = fmtDateFull('2024-03-15T18:30:00Z', 'America/New_York')
+    expect(result).toMatch(/2:30/)
+    expect(result).toMatch(/EDT/)
+  })
+
   it('returns — for null', () => {
     expect(fmtDateFull(null)).toBe('—')
   })