On Sun 2021-05-23T22:43:46+0100 Clive D.W. Feather via tz hath writ:
> I would actually say that, if we're going to do something like that, there
> should be a way to put a "not valid before" marker into the zone data and
> have the functions return either an error or a clearly nonsense answer.

The disruption caused by creating such an interface and the
maintenance of the cutoff dates would be two reasons to say that what
LMT values are in existence now already implements this idea.