Create a timestamp field called edited and create an auto-enter calculation on it with:
Case ( PrepStatus = "Edit!" ; Get ( CurrentTimeStamp ) )
Be sure to uncheck 'Do not replace existing value in field if any' which is right below the Calculated Value on the auto-enter tab. In this way, when User selects Edit! a new timestamp will enter the edited field but will clear itself whenever the value changes to something else.
RentalPrepStatus = "Edit!" ; Get ( CurrentTimeStamp )