Right - date-times in the past are always easy (at least until you have to take relativity into account). An event happened at some instant in the universe and you just need an agreed upon representation of that instant. UTC works fine for this - record the UTC-based instant at which the event happened and you can always translate it into any other representation without losing information. Recording it in your local timezone is fine too, as long as you also record the UTC offset or timezone along with the instant.

> I guess it should be detectable which dates may be ambiguous/wrong if you know the date of their creation (before the change was known)

Yeah - in theory, when a timezone is added, you could probably link it to timezones that users of the new timezone might have previously used. And then any future times that were saved using that timezone, you ask someone if they are still correct or if the timezone needs to be adjusted to the new one

For example, if a new timezone was added for southeast Colorado, you might ask someone about all times scheduled in both the Denver & Phoenix timezones, because you don't know which one people might have picked.

It gets complicated though because you need to keep track of which entries have been double checked and which ones haven't, and you need to keep track of the version of tzdb that you reconciled against, because there could be another change in the future.