diff --git a/lib/agent/dave-presence.mjs b/lib/agent/dave-presence.mjs
index 5dee4eb..c069f0f 100644
--- a/lib/agent/dave-presence.mjs
+++ b/lib/agent/dave-presence.mjs
@@ -78,8 +78,12 @@ export class DavePresence {
     const profile = this.profileStore?.getAgentProfile();
     if (!profile) return this._result(false, 'profile_missing');
 
-    const timezone = profile.timezone || this.fallbackTimezone;
-    const clock = localClock(now, timezone);
+    let timezone = profile.timezone || this.fallbackTimezone;
+    let clock = localClock(now, timezone);
+    if (!clock && timezone !== this.fallbackTimezone) {
+      timezone = this.fallbackTimezone;
+      clock = localClock(now, timezone);
+    }
     if (!clock) return this._result(false, 'invalid_timezone');
     this._rollDay(clock.day);
     if (this.state.sentCount >= this.maxPerDay) return this._result(false, 'daily_limit');
diff --git a/test/dave-presence.test.mjs b/test/dave-presence.test.mjs
index a5decb9..830154b 100644
--- a/test/dave-presence.test.mjs
+++ b/test/dave-presence.test.mjs
@@ -89,3 +89,11 @@ test('local clock uses the profile timezone', () => {
   assert.deepEqual(localClock(new Date('2026-07-06T12:30:00Z'), 'Europe/Berlin'), { day: '2026-07-06', time: '14:30' });
   assert.equal(localClock(new Date(), 'Invalid/Timezone'), null);
 });
+
+test('invalid profile timezone falls back to configured operational timezone', async () => {
+  const { presence, messages, calls } = setup({ profile: { timezone: 'not-a-timezone' } });
+  const outcome = await presence.tick(new Date('2026-07-06T12:00:00Z'));
+  assert.equal(outcome.reason, 'sent');
+  assert.equal(messages.length, 1);
+  assert.match(calls[0].prompt, /timezone: UTC/);
+});