How to design a calendar?

Question

Software design

It is necessary to implement the functionality of the calendar and recurring reminders. And my brain explodes.
How to store dates? In the user's local time zone or in unix timestamp? What if the time zone changes? Due to a law changing local time, or if the user will be in a different zone at the time of the event?
How to store recurrent events? Every time dynamically calculate? But what if the user wants to change one of the events? And if there is an end date for recurrent events, can they be created immediately in the database?
And something tells me that all these problems have already been solved many times, but I can’t google how large calendars like Outlook / Google calendars are designed.

Answer 1

googled how libraries are designed,
for example
https://dateparser.readthedocs.io/en/latest/
everything is simple there - normalization to UTC and checking of boundary states between days and dates