... & a routine for parsing the UTC leapseconds file.

But mostly Hare's `time::chrono` standard library supplies that `timescale` type holding a name, abbrevation, & conversion callbacks.

It has a routine for iterating over the given timescales & converting converting an argument to it, trying multiple conversion callbacks.

There's timescales defined for TAI (no leapseconds), UTC (has leapseconds, GPS (different epoch than UNIX), Teresstrial Time (different epoch), ...

3/4!!