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 line endings are \r\n

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

an all day event can span over multiple days and look like this:
DTSTART;VALUE=DATE:20230911^M$
DTEND;VALUE=DATE:20230916^M$

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

appointments can also span over multiple days:
DTSTART;TZID=/freeassociation.sourceforge.net/Europe/Berlin:
 20230913T230000
DTEND;TZID=/freeassociation.sourceforge.net/Europe/Berlin:
 20230914T040000

Appointments include the continent and city of the used time zone.

"DESCRIPTION:" is an optional field.