Custom validation of 'To' and 'From' dates - any ideas?

Simply put, I'd like to check if the 'From date' is after the 'to date' and if it is, instead of throwing an error message, Id like to set the 'From date' equal to the 'To date'.

So I'm looking for a way of achieving this without hacking the module, which means performing my validation before the date module performs its check (and resultant error message)

I tried configuring a rule that fires when 'content is about to be saved' using Rules module, but the Date module seems to get there first with its error message. I'd be grateful for any other pointers.