Convert Date - 8/29/2011 to 2011.08.29

Hello,
Can anyone help me with a creating a formula that would change the following date format? I attempted to make the changes with Excel's date format but it doesn't recognize the dates. I'm thinking a substitute formula would work best but I'm not sure. Below is an example of how I want to convert the date.

8/29/2011 to 2011.08.29

Thanking you for taking the time to read this post and for any and all help you can provide.