This is a change to the API, but existing users are already inconsistent
in how they handle 2-digit years -- some assume it's year % 100 (as the
documentation says), some assume it's year - 1900, and some
string-prepend "19" -- so most have Y2K problems that need fixing.
As it's now reading the date using two UUOs, it needs to be careful to
check that the year hasn't rolled over between the two, as LIBDOC; TIME
does for the day.
I've updated all four implementations of SDATE, but only tested the ITS
version.
