I just need to change the encoding date in an mp4

Hi
I'm a total video noob as I work in SEO but I need to change the meta data in a video file so that Google will read a recent date.

If I inspect the mp4 files with an application I found (MediaInfo) it shows this

Encoded date : UTC 2011-08-10 22:09:14
Tagged date : UTC 2011-08-10 22:09:14

unfortunately the application only reads the data and won't let me change it

does anyone know of a very simple (and free, if possible) app that can let me update this info without having to re-encode the video files ?

thanks for any help you can give