There is no easy way to format just the year in a PPT date footer.
This is a workaround. My use case is a copyright stamp in the lower right hand corner of each slide. I have one template with my house style. For new presentations, I edit the template. For existing presentations that you need to update, skip the first step.
For PTT 2010, your templates are in
C:\Users\<user>\AppData\Roaming\Microsoft\Templates. '..\Templates' may be the word for Templates in your own language. YMMV, Microsoft tends to change its mind about the best place for these with various versions of PPT and Windows. Right click on the template and select Open. Don't doubleclick, you'll open a new blank document from the template. That's not what you want.
Now open the View tab and select 'Master slides', or Microsofts' translation for it ('Diamodel' in Dutch).
In the left pane, you see the master-master, slightly larger than other style masters below it. Click it.
Click anywhere on the slide in the main panel, go to the Insert tab and make a nice text box with your year in it. Place it where it doesn't sit in the way of anything else, like headers or footers. I'm not using any headers or footers, so I put mine in the lower right corner, partially overlapping with the default footer box for slide number.
If you did step 1, save your template and start a new presentation from it. (File -> new -> your bewdiful template.)
Next year, remember to check back here and repeat. I hope I will. Re-invented this wheel at least three times in the last few years.