How do I add validation a form?

I highly recommend moving to a more recent version of Appian. 7.3 hasn't received patches in about a year, so you likely have unresolved security vulnerabilities.

Basically I need a validation to be done for the date field(to check if the input date is lesser than the process starttime) since JavaScript is not supported how can I achieve it in the form?

Form here we are not using an interface instead the usual form .

Thanks in avance