Is there a way to show the date (in addition to time) in the OS X lock screen?
Another option, which worked for me, was the following crontab entry, in root's crontab:

0 0 * * * defaults write /Library/Preferences/com.apple.loginwindow LoginwindowText "The date is `date '+%B %d, %Y'`"


This sets a message on the lock screen, including the date.  It isn't as slick as including the date with the time in the corner, but it has the advantage of working at all (I couldn't get @konqui's answer to work for me), and it doesn't require installing any 3rd-party software.

If anyone is interested, I detailed the procedure in more depth on my blog, but the essence of the solution is posted here.