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).

Lines of text SHOULD NOT be longer than 75 octets, excluding the line
break.  Long content lines SHOULD be split into a multiple line
representations using a line "folding" technique.  That is, a long
line can be split between any two characters by inserting a CRLF
immediately followed by a single linear white-space character (i.e.,
SPACE or HTAB).  Any sequence of CRLF followed immediately by a
single linear white-space character is ignored (i.e., removed) when
processing the content type.

For example, the line:

 DESCRIPTION:This is a long description that exists on a long line.

Can be represented as:

 DESCRIPTION:This is a lo
  ng description
   that exists on a long line.

The process of moving from this folded multiple-line representation
to its single-line representation is called "unfolding".  Unfolding
is accomplished by removing the CRLF and the linear white-space
character that immediately follows.

When parsing a content line, folded lines MUST first be unfolded
according to the unfolding procedure described above.

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Observation of different ics files from different sources
---------------------------------------------------------

ICS files from Microsoft Outlook differ from those that were created by Gnome Evolution.
The order of "fields" seems not be standardized and differs between every application.


Evolution
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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.