This is the Coptic Orthodox Church calendar,
that starts the 11th or 12th September and has
13 months.
Note
The (default) names of the months
are different then other the emacs calendar;
I do not know which ones are used - the difference
seem to be only the transcription of the phonetic sounds
(B <-> P, etc).
I do not know for how long back the calendar is valid,
either. My sources claim that the calendar is synchronized
with the Gregorian calendar, which is odd.