I guess that I'd never make the assumption that the user made a reversal of the dates even by accident. What if they phat phingered something and they don't notice and then use that report as some form of gospel?
Protect the user-in-a-hurry from such an eventuality. Make the report (or what have you) error out with an informative reason. Having the user correctly enter the dates in the correct order is a form of validation that I wouldn't gloss over.
--Jeff Moden
Change is inevitable... Change for the better is not.