I am new to mocking so I might have it totally wrong here but I believe that most mocking frameworks are interface dependent. Unfortunately most of our code is not using an
Microsoft Research has developed Moles for this, which is a part of Pex but can be installed independently. And it's free. There's a good introductory article (pdf) on the website that explains how to mock a static method. It takes some time before they get to the stuff you want (page 16, Task 3).
Here and here (Channel 9) you can find an example on how to stub DateTime.Now. Using Moles, you can mock anything you want.