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

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))});