It seems that the Optiplex 960 has the Core 2 Quad processor. That came out around quarter 1, 2007.
So, your machine was probably first sold in 2007 or 2008.
The 9010 seems to have 2nd and 3rd generation processors. So, you can find your processor on this list and then assume that the computer could have been sold something like within 2 years after that