Usually, through a workflow validator. If you have a validator (might need a plugin I'm afraid) there then it validates on "Create" button. Meaning if the date is not valid, then the user gets an error message (usually customizable to indicate the field w/ message), forcing them to change the value before trying again.
Out of box I don't think you can validate custom fields, but perhaps they added something in data center that I don't recall. Look around the validator options though if it contains something you could use.
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