True, but also for the past, actually. Because DST might be a thing right now in that zone, but may no longer be next year, in which case the historical DST is

True, but also for the past, actually. Because DST might be a thing right now in that zone, but may no longer be next year, in which case the historical DST is needed to refer to times in the past. Same for dates a bit further back when countries changed calendars and some days are missing. So, we basically need a mapping of UTC -> local time as a function of time, and store this forever. (For durations, we might need to have a mapping from TAI to UTC as a function of time, because a leap second messes with duration length. Smeared leap seconds are even worse in that regard)