ICS files from Microsoft Outlook differ from those that were created by Gnome Evolution.
(semicolons are used instead of colons at some position)

Evolution
---------
all day event:
DTSTART;VALUE=DATE:20230908
DTEND;VALUE=DATE:20230909

appointment:
DTSTART;TZID=/freeassociation.sourceforge.net/Continent/City:
 20230909T090000
DTEND;TZID=/freeassociation.sourceforge.net/Continent/City:
 20230909T093000

This includes the continent and city of the used time zone.