RFC-219 as follows:
- DateTime(string, timescale) with "Z" required for UTC and forbidden for TAI. Ideally forbid "23:59:60" leap seconds for TAI as well.
- DateTime.toString(timescale) with "Z" produced for UTC and not for TAI.
Require the timescale argument, unless it proves to be a major headache by affecting too much code, in which case default to UTC for backwards compatibility.
Additional tests that verify that "23:59:60" strings are not produced by toString(TAI) at actual leap seconds.