date time stamp when a user fills in field on a form

This is quite tricky to implement without having to embed a code that does it in all the fields in the file. The closest I got to it is by using this code as the custom calculation script of the field where you want this date to appear. You can set the field as read-only and then you'll have to reset the form in order to clear it.

if (event.source && event.target.value=="") {
    event.value = util.printd("mm/dd/yyyy HH:MM", new Date());
}