Yes - you need to knew will it be IWP or not to create great layouts.
In your case probably better is to use Validate field content - as I understand the only task is to check is Date correct or not.(via calculation).
You always need to keep in mind - IWP is for simple layouts (for example - no tab panels) and minimum script triggers.
Thanks Peter. I sort of had a feeling that was the case (simplicity) and wish I would have known IWP would have been used as to not put all of the work into scripts. Such is life.
Thank you for your reply. I will use calculations in validation.