Here is a cool looking draft that seems to be based on the current datepicker.
The jqueryui date picker could become a datetime picker like the one above. And then the options could specify if you want just date, date and time, or just time.
There is a strong need for one of these. The user experience with using select form fields on Month, Day, Year Hours:Minutes:Seconds is very poor because of the number of clicks required to fill it out. This widget could become a very big hit if it had dual functionality.