RFC5545
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https://www.rfc-editor.org/rfc/rfc5545

The iCalendar object is organized into individual lines of text,
called content lines. Content lines are delimited by a line break,
which is a CRLF sequence (CR character followed by LF character).


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$

The DTEND of an all day event is exclusively, it is not an all day event anymore!

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

SEQUENCE is the number of times the event was modified.
Evolution sets this to 2 after it has been generated.

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

"DESCRIPTION:" is an optional field.