diff --git a/agenda/calendar.py b/agenda/calendar.py
index 2dd7978..f037a3c 100644
--- a/agenda/calendar.py
+++ b/agenda/calendar.py
@@ -1,8 +1,8 @@
 """Calendar."""
 
 import typing
-from datetime import timedelta
 import uuid
+from datetime import timedelta
 
 from .event import Event
 
@@ -89,7 +89,15 @@ def build_toastui_events(events: list[Event]) -> list[dict[str, typing.Any]]:
         if e.has_time:
             end_iso = (e.end_date or e.date + timedelta(minutes=30)).isoformat()
         else:
-            end_date = (e.end_as_date if e.end_date else e.as_date) + one_day
+            # For all-day events, the end date is exclusive.
+            # For a single-day event, the end date should be the same as the start date.
+            # For a multi-day event, a day is added to make the range inclusive.
+            if e.end_date:
+                # This is a multi-day event, so add one day to the end.
+                end_date = e.end_as_date + one_day
+            else:
+                # This is a single-day event. The end date is the same as the start date.
+                end_date = e.as_date
             end_iso = end_date.isoformat()
 
         item = {