Option 3: preserve local time, using UTC as derived data to be recomputed

Spoiler alert: this is my preferred option.

In this approach, the information that the conference organizer supplied (“9am on July 10th 2022”) is preserved and never changed. There is additional information in the entry that is changed when the time zone database is updated: the converted UTC instant. We can also preserve the version of the time zone rules used for that computation, as a way of allowing the process of updating entries to be restarted after a failure without starting from scratch, but it’s not strictly required. (It’s also probably useful as diagnostic information, too.)

The UTC instant is only stored at all for convenience. Having a UTC representation makes it easier to provide total orderings of when things happen, and also to compute the time between “right now” and the given instant, for the countdown timer.