Having moved home I've been sorting through boxes of old junk working out what I could throw away or at least what I could store in the loft. One of the boxes contained a whole bunch of old University paperwork.
One of the pieces of paper concerned my account on the department of computer science's teaching network. I never noticed before but I'm guessing that the expiry date of 23.09.101 was caused by the millennium bug. For those of you who can't remember what all the fuss was about computers had been using just two digits to store the year which had worked well for most things but suddenly they would need to use four digits or the world would end! See Wikipedia if you really want all the gory details.
I'm guessing the department's system got fixed as my account continued to work correctly all the way to 23.09.2001.