Dennis @ PerfectForms
Last submission date is meta-data which makes it easy to use in reports.
Most people who need date/time stamps use hidden helper fields and a behavior on the button used for submit. Before the Submit object use Set Field objects to set a helper date field and a helper time field. If you need to keep a series of dates and time you could use a hidden helper table with a time column and a date column. When the submit button is pressed add a row to the table and set the data and time fields (current row).