Some bulletproofing on bad input to Unix as a setter

Since you can technically say
    $date->Unix(-5);
and
    $date->IsSet
would return 1
    but $date->ISO
would return 1970-01-01 00:00:00
we should bulletproof for insane inputs.

This lets IsSet just assume that unix time starts at 0 as intended, and
avoids all the code shenanigans removed in the previous commit which
checked for $date->Unix <= 0  or $date->Unix < 1