iCalendar

iCalendar promo

The events in your calendar are most likely stored in iCalendar (.ics) format.
https://en.wikipedia.org/wiki/ICalendar

The recurrent rule (RRULE/RECUR) format was invented to describe how often an event recurs. RRule itself is pretty flexible, allowing you to set rules like “every Saturday in January at 8:30 and 9:30 am every other year: RRULE:FREQ=YEARLY;INTERVAL=2;BYMONTH=1;BYDAY=SU;BYHOUR=8,9; https://tools.ietf.org/html/rfc5545

For RRule parsing, I found several libraries:

2020-10-14 Edit icalendar.md