> if it does come with a timezone, then there is no issue with that future shift.

There is no issue that didn’t already exist, but it remains true that future wall-clock times cannot be predicted with certainty, because tzdata may change between now and the future time.

When someone sets an alarm for 12:00 in five years in the current time zone, how do you store that? It can make a difference whether you store it as a Unix timestamp or as broken-down time plus time zone.