[eluser]oddman[/eluser]
Interesting post, as I've been wondering how to get callbacks to make use of more than one field (at this point it seems impossible without some code hacks of the Form_Validation library, easily extended, however
)
I would either:
a) Force the user to put it in one field, and give them a format example (ie. dd-mm-yyyy) or
b) Use a date picker as SeasonedCoder suggested
my 2c.