I think it's silly to throw an error for dates before 1972. We could display `1970-01-01T00:00:00 GMT` for dates before 1972, or even not display any abbreviation, but that all seems a little far-reaching. My vote is that we use proleptic UTC. I also don't see any reason to mess with things in the future. If using `UTC` makes anyone feel to0 queasy or uneasy, then we can change everything to `UT` or `GMT` to denote that we're using a sensible default.