Automatic field to display date saved/modified

Hello,

I have a "Revision Date" field in a footer but it displays the current date. I would like to display the date the file was last modified.  This script in the layout:ready event is:

this.rawValue = util.printd("yyyy/mm/dd", new Date());

I'm looking for an equivalent of the modDate property in Acrobat, but the Javascript reference says there's isn't one for LiveCycle.

Any help is appreciated, thanks!