Whenever you see a UTC time, the timestamp itself tells you how which "leap second epoch" it falls into (with the exception of instants that fall within the lea

Whenever you see a UTC time, the timestamp itself tells you how which "leap second epoch" it falls into (with the exception of instants that fall within the leap second itself). The only extra information you need to correctly do time calculations is the table of leap seconds, and that's a very small amount of mostly-static data that everyone can agree on. With your proposal, time handling would be slightly simpler, but every geographic coordinate everywhere would need to be tagged with a date to let you know which epoch it was relative to, or else you would get huge (in local terms) errors.