Do you want to do it with WebAssist tools? Or, the hard way where you have to use the explode command to get the various date parts split apart and then re-apply them into variable the correct order.
If you have Validation Toolkit, you can apply a client validation and define the accept value and then in the same dialogue box, it allows you to pick a format to convert it to. This method, however, does use javascript to reformat the date.