IF there's any chance that your db will be used with a different version of Access than the one you're developing it in, heeding the good doctor's advice would be wise. If you still want to use the ActiveX Calendar, code something like this will do it
Code:
Private Sub ActiveX_Calendar_AfterUpdate()
If Me.ActiveX_Calendar < Date Then
MsgBox "You cannot pick a date in the past! Try again!"
Else
'Code here to assign your date to your textbox
End If
End Sub
I have heard lots of horror stories about folk who have tried to use the ActiveX Calendar control and just had major issues with transfering their database form the development machine to other users.
I use the one that I posted the link to all the time and never have any issues with it. That is why I reccommended it here.