Since you are already using a script, you don't need a custom function, but such could be used.
Here's a script that could check to see if the date is greater than a specified minimum acceptable value:
If [ IsValid ( YourTable::DateField ) and YourTable::DateFIeld > Date ( 1 ; 1 ; 1950 ) // use whatever date best works for this here ]
# Do whatever you need to do if the date is valid here
# Do whatever you need to do if the date is invalid here
pff...I should have known it was that simple...
Thanks a bunch!