fix: tolerate invalid device timezones in Week Ahead (Closes #1177)

frontend/today-rollover.js

@@ -4,7 +4,13 @@ function createTodayRollover(options = {}) {
     Intl.DateTimeFormat().resolvedOptions().timeZone || 'UTC');
 
   function timeZone() {
-    return options.timeZone?.() || resolvedTimeZone();
+    const candidate = options.timeZone?.() || resolvedTimeZone();
+    try {
+      new Intl.DateTimeFormat('en', { timeZone: candidate }).format(now());
+      return candidate;
+    } catch (_error) {
+      return 'UTC';
+    }
   }
 
   function localDate() {

tests/test_today_rollover.py

@@ -132,6 +132,18 @@ process.stdout.write(JSON.stringify({{date:rollover.localDate(),zone:rollover.ti
     assert run_node(script) == {"date": "2026-11-01", "zone": "America/New_York"}
 
 
+def test_invalid_runtime_timezone_falls_back_to_utc_without_breaking_planners():
+    script = f"""
+const create = require({json.dumps(str(ROLLOVER))});
+const rollover = create({{
+  now:()=>new Date('2026-08-20T23:30:00Z'),
+  resolvedTimeZone:()=> 'Etc/Unknown'
+}});
+process.stdout.write(JSON.stringify({{date:rollover.localDate(),zone:rollover.timeZone()}}));
+"""
+    assert run_node(script) == {"date": "2026-08-20", "zone": "UTC"}
+
+
 def test_rollover_is_one_durable_sync_operation_with_the_server_revision():
     script = f"""
 const createSync = require({json.dumps(str(SYNC))});