A time and Date Formula

[sinspawn56]

Hello.
Here is what I want to do.
I have a centralized document that myself and my co-workers use. I would like to insert a date formula that upon opening the "Main" document displays the current date (that is no problem). The catch is, when myself or my co-workers make a change to that document, and "save as" I would like that date to stick (no longer display the current date, but the date in which it was "saved as").

Ideas?