diff --git a/py/icalfilter/app.py b/py/icalfilter/app.py
index a42b21adb3..a432409ba8 100644
--- a/py/icalfilter/app.py
+++ b/py/icalfilter/app.py
@@ -3,8 +3,8 @@ import concurrent.futures
 import dataclasses
 import datetime
 import json
-from typing import Any, Dict, List, Optional, Set, Union
 import os
+from typing import Any, Dict, List, Optional, Set, Union
 
 import aiohttp
 import attrs
@@ -31,7 +31,10 @@ def maybe_fromisoformat(val: Optional[str]) -> Optional[datetime.datetime]:
         return None
     if isinstance(val, datetime.datetime):
         return val
-    return datetime.datetime.fromisoformat(val)
+    dt = datetime.datetime.fromisoformat(val)
+    if not dt.tzinfo:
+        dt = dt.replace(tzinfo=UTC)
+    return dt
 
 
 @attrs.frozen
@@ -97,15 +100,11 @@ def _all_occurrences_before_expensive(
     seen_timezones: Dict[str, icalendar.Timezone],
 ) -> bool:
     # Recurring events are... more complicated.
-    # Generate an iCal of just this event...
-    fake_cal = icalendar.Calendar()
-    for tz in seen_timezones.values():
-        fake_cal.add_component(tz)
-    fake_cal.add_component(event)
-
-    # and check to see if we get any instances that are after the cutoff.
-    instances = icalevents.icalparser.parse_events(fake_cal.to_ical(), start=cutoff)
-    return not instances
+    rrule_or_rruleset = icalevents.icalparser.parse_rrule(event)
+    try:
+        return not rrule_or_rruleset.after(cutoff)
+    except TypeError:
+        return not rrule_or_rruleset.after(cutoff.replace(tzinfo=None))
 
 
 async def all_occurrences_before(