Month/year views incorrectly create new events as UTC instead of current time zone
Calendar currently converts times to UTC for DATE-TIME values in iCalendar VEVENTs instead of the current time zone.
This is not completely equivalent to the time the user entered in reference to their time zone. Conversion to UTC throws away information.
Example 1 - future time zone redefinitions:
A user sets an event for 16:00 1st December 2020, Europe/London. Currently, that is expected to be 16:00 UTC. So Calendar saves it as T160000Z. However, imagine if in the meantime, daylight savings time (DST, = UTC) is abandoned (something that is quite possible and is periodically discused in the UK). So then, 16:00 1st December 2020 Europe/London will actually be be UTC+1, i.e. T150000Z. So Calendar, in throwing out time zone information, will get this date wrong.
Example 2 - recurrence:
A user sets an event to recur over a period that crosses a change in UTC offset in a time zone (e.g. summer time to winter time). With the event set to UTC, the time of the event local to that time zone will change when going from e.g. summer time to winter time. In the UK for instance, it will be an hour earlier in winter.
Although Calendar 3.24 doesn't support recurrence (I belive 3.26 will?), adding recurrence elsewhere in a well-behaving client* after creating the event in Calendar will trigger this bug.
In short: use the current time zone when writing out an event to iCalendar.
*Evolution is not currently well-behaving - it converts to the local time zone when editing a UTC event.