I haven't found any way to do this except after the fact, i.e., I can check the value after the form has been submitted and send the user an email saying their selections were invalid, but this is terrible user experience and I'd rather avoid it.
The form doesn't seem to have any ability to interact with dates other than comparing them against static dates that you set in rules, which is not really useful at all.
I'm trying to allow users to select start dates that are not in the past, and end dates that are a minimum of 30 days after their selected start date.
