Fix #28104 - CalDav support for floating datetimes

Timzones are optional in CalDav

It is possible that an entry contains neither a TZID, nor is an UTC time.
When this is the case, it should be treated as a floating date-time value,
which represent the same hour, minute, and second value regardless of which
time zone is currently being observed.

For Home-Assistant the correct timezone therefore is whatever is configured
as local time in the settings.

See https://www.kanzaki.com/docs/ical/dateTime.html

homeassistant/components/caldav/calendar.py

@@ -262,9 +262,12 @@ class WebDavCalendarData:
     @staticmethod
     def is_over(vevent):
         """Return if the event is over."""
-        return dt.now() >= WebDavCalendarData.to_datetime(
-            WebDavCalendarData.get_end_date(vevent)
-        )
+        end_date = WebDavCalendarData.to_datetime(WebDavCalendarData.get_end_date(vevent))
+        if end_date.tzinfo is None:
+            # floating value, not bound to any time zone in particular
+            # represent same time regardless of which time zone is currently being observed
+            end_date = end_date.astimezone(dt.DEFAULT_TIME_ZONE)
+        return dt.now() >= end_date
 
     @staticmethod
     def get_hass_date(obj):