The Document Object Model that we manipulate in JavaScript includes a property that identifies the date and time the current document was changed. If, in your code, you make a reference to document.lastModified, you'll get a Date object (which you can just print as a text string) indicating the relevant point in history. 

A simple usage of the property looks like this:

document.write(document.lastModified)

Though precise formats vary among browsers and localities that code will yield something like this:

08/12/2002 17:36:32

That is, it returns a mm/dd/yyyy date and hh:mm:ss time. The date and time come from the operating system, and usually won't change if you open, save, and close a file without modifying it. You can extract bits and pieces of the date and time easily because it's a Date object. Use the Date.getDate(), Date.getMonth(), and Date.getYear() methods to pull out the pieces you want and reassemble them as you wish.

You'll find an application that uses the document.lastModified property, complete with a utility that formats the date nicely and gets around turn-of-the-millennium problems, here: http://www.chami.com/tips/internet/041198I.html