diff --git a/frontend/tomorrow-plan.js b/frontend/tomorrow-plan.js
index fd29455..5657984 100644
--- a/frontend/tomorrow-plan.js
+++ b/frontend/tomorrow-plan.js
@@ -27,8 +27,15 @@ function createTomorrowPlan({fetchJson,localDate,timeZone}={}) {
   function millisecondsUntilNextDay() {
     const now=Date.now();
     const zone=timeZone();
-    const formatter=new Intl.DateTimeFormat('en-CA',{timeZone:zone,year:'numeric',month:'2-digit',day:'2-digit',
-      hour:'2-digit',minute:'2-digit',second:'2-digit',hourCycle:'h23'});
+    let formatter;
+    try {
+      formatter=new Intl.DateTimeFormat('en-CA',{timeZone:zone,year:'numeric',month:'2-digit',day:'2-digit',
+        hour:'2-digit',minute:'2-digit',second:'2-digit',hourCycle:'h23'});
+    } catch (_error) {
+      const next=new Date(now);
+      next.setHours(24,0,0,250);
+      return Math.max(1,next.getTime()-now);
+    }
     const zoned=value=>Object.fromEntries(formatter.formatToParts(new Date(value))
       .filter(part=>part.type!=='literal').map(part=>[part.type,Number(part.value)]));
     const current=zoned(now);
diff --git a/tests/test_tomorrow_plan_frontend.py b/tests/test_tomorrow_plan_frontend.py
index 65ad591..1a621a1 100644
--- a/tests/test_tomorrow_plan_frontend.py
+++ b/tests/test_tomorrow_plan_frontend.py
@@ -124,6 +124,20 @@ console.log(JSON.stringify({checks,delays:timers.map(timer=>timer.delay),cleared
     assert result["cleared"] == [1, 2]
 
 
+def test_tomorrow_rollover_falls_back_when_the_reported_timezone_is_invalid():
+    result = run_controller("""
+let scheduled=null;
+const planner=createTomorrowPlan({fetchJson:async()=>({}),localDate:()=> '2026-08-20',timeZone:()=> 'Etc/Unknown'});
+const lifecycle=planner.startLifecycle({
+  check:async()=>false,setTimer:(_fn,delay)=>{scheduled=delay;return 1;},clearTimer:()=>{},
+});
+await lifecycle.run();
+console.log(JSON.stringify({scheduled:Number.isFinite(scheduled)&&scheduled>0}));
+""")
+
+    assert result == {"scheduled": True}
+
+
 def test_tomorrow_plan_has_a_touch_safe_mobile_entry_and_reuses_the_ordered_planner():
     index = INDEX.read_text()
     css = CSS.read_text()