Each time form is saved, it should have with new date in chronological order

My question is little bit tricky for me but could be easy for you. I have a  Livecycle form and I am trying to create a script which will save the form with date that means each time a form is either changed or saved it saved with a new date stamp you can say a log of dates. The date needs to insert under the last Item shown so I can see when each round of changes was submitted The date field is in subform and it save current date and time but once only, I want to save it as many times as it is saved or submitted. The formula I put is this.resolveNode("$").rawValue = util.printd("dd/mm/yyyy HH:MM:ss", new Date()); and it works for one time only. Kindly help. Screenshot attached it saved once only. Thanks