... which is the format expected/required when entering or editing birthdates
in the terminal server. So this is a pretty embarassingly bad bug introduced
just before the v3.20b release to fix a less-severe bug with ecWeb new user
registration (still) writing 2-digit years to the user.birthdate field. :-(

Fix for issue #863

I considerd making these get_birthdate field functions more tolerant of less
strictly formatted strings (e.g. single-digit month or day), but decided to
keep this change to just a fix for the issue at hand. These functions could be
more robust to handle more variance in user/script input.