Struct UnixEpochSeconds

The supported range of seconds for the difference between any two values of UnixEpochDays.

This range should correspond to the first second of Year::MIN (with a minimal offset) up through (and including) the last second of Year::MAX (with a maximal offset). Actually computing that is non-trivial, however, it can be computed easily enough using Unix programs like date:

$ TZ=0 date -d 'Mon Jan  1 12:00:00 AM  -9999' +'%s'
date: invalid date ‘Mon Jan  1 12:00:00 AM  -9999’
$ TZ=0 date -d 'Fri Dec 31 23:59:59  9999' +'%s'
253402300799

Well, almost easily enough. date apparently doesn't support negative years. But it does support negative timestamps:

$ TZ=0 date -d '@-377705116800'
Mon Jan  1 12:00:00 AM  -9999
$ TZ=0 date -d '@253402300799'
Fri Dec 31 11:59:59 PM  9999

With that said, we actually end up restricting the range a bit more than what's above. Namely, what's above is what we support for civil datetimes. Because of time zones, we need to choose whether all Timestamp values can be infallibly converted to civil::DateTime values, or whether all civil::DateTime values can be infallibly converted to Timestamp values. Jiff choses the former because getting a civil datetime is important for formatting. If Jiff didn't choose the former, there would be some timestamps that could not be formatted. Thus, we make room by shrinking the range of allowed instants by precisely the maximum supported time zone offset.