Why do thousands of my files and emails have the date August 31, 2007 and how do I fix the problem?

My iMac circa 2008 model stamps documents, emails and anything its little hands can grab with the date August 31, 2007. I running latest Mountain Lion version and this problem goes back to Tiger, Leopard, Snow Leopard and Lion as well.

I just sent a Photo Booth picture to a friend. On my screen and my sent mail it had the correct date. The recipient called me to ask why it was dated August 31, 2007.

There must be a simple fix. Can anyone help. I've let this bug drive me crazy long enough.