JotForm is a free online form builder which helps you create online forms without writing a single line of code. No sign-up required.
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I am not sure if I have understood your question correctly.
Do you mean to ensure that the input value of a form field is actually a date? In that case you should go ahead and add "DateTime" field in your form which in under "Quick Tools". Please check the screenshot below:
Hope this helps.
Do get back to us if you meant to ask something else.