Add date/time of editors save

How can I automate the process of recording the date/time when a section was edited. Currently I'm using,,,

<perch:content id="course_status_time" type="date" time="true" label="Time of Update" format="%d %B %Y : %H %M %P" help="Edit the time of this update" />

Which would create the current time if I was starting a new section but would not save the edit/save time.

This is prone to errors by the editor when they come back for updates when just the updated time on save is needed automatically.

Any ideas how?