Userdefinedforms seem to allow date fields but not time or date-time.
Is this easy to add? E.g. the date field could have an option to either be "date-only, datetime, or time-only" (rather than clutter up the addfield option with lots of similar field types).
Also, a tickbox for 'default to current date/time' would seem very sensible, and you would tick it by default :)