You can have a field in your table for LAST UPDATED, e.g. dtmLastUpdate. In your form's AFTER UPDATE event (not a field's AFTER UPDATE event, the form itself), you can then put something like
Me.txtLastUpdate = Now()
That will put the day and time in that field. That textbox, txtLastUpdate, would be bound to the table's field dtmLastUpdate.
Michigan Dept. of Environmental Quality, Air Quality Division