[spenczar5]

UTC is non-unique because of leap seconds, so TAI + lat + long + altitude is actually required.

I work on software for astronomy, and that quadruplet is what’s used to describe an observing location. You can actually get a little in trouble because of changes in the shape of the earth over time, so latitude and longitude and altitude need to be treated as time-dependent values, which matters once you are accounting for relativistic effects.

[agalunar]

UTC times are unique: when a positive leap second is inserted, 23:59:60 is added; when a negative leap second is inserted, 23:59:59 is skipped.

Unix timestamps, on the other hand, are not unique.