time-util: refuse formatting/parsing times that we can't store

time-util: refuse formatting/parsing times that we can't store

usec_t is always 64bit, which means it can cover quite a number of
years. However, 4 digit year display and glibc limitations around time_t
limit what we can actually parse and format. Let's make this explicit,
so that we never end up formatting dates we can#t parse and vice versa.

Note that this is really just about formatting/parsing. Internal
calculations with times outside of the formattable range are not
affected.