Make (Utc)DateTimes sortable

[Kijewski]

No description provided.

[x-hgg-x]

DateTime cannot implement PartialOrd by comparing Unix times since it won't be consistent with its PartialEq implementation.

[Kijewski]

Do you think a custom implementation of PartialEq for DateTime would be OK if it only compares the Unix times and time zones? The other fields are derived data, aren't they? (I guess I missing some finer parts like the handling of leap seconds or such.)

[x-hgg-x]

Yes, this PartialEq implementation is ok since the leap seconds are normalized and not preserved. But then we need to also compare the Unix times and time zones for the PartialOrd implementation (not just the Unix times) in order to have consistent implementations.