[eluser]TheFuzzy0ne[/eluser]
Hi there. I had an epiphany whilst walking to my friends house, and I think I've found you're problem. You're using the validation library, which is deprecated. You should be using the form_validation library. I'd also suggest you recheck your callback logic to make sure the date is actually valid. Here are a few examples of dates that would be valid with your callback:
00-00-00 - Obvious

32-13-00 - No month has 32 days (although I wish they did, since there's never enough time).
30-02-09 - February never has 30 days
I'd suggest you add some extra checking in there to ensure that the date is valid.
Hope this helps.