validate date fields before saving

I want to record the dates on which a boat will be leaving and returning to their dock.  I don't want the return date to predate the departure date.  I see how I can use a dynamic message to warn the user of invalid dates, but it lets them save it anyways.  (Seems to me that "prevent save" would be a good action to include in dynamic forms.)  But that doesn't seem to be an option.  I thought maybe a formula field holding the duration between the two dates would also work, but I don't see how the calculated field would be validated.  Am I missing something, or is this a glaring gap in the product design?