Here's another idea:
Continue using the same tab in the same spreadsheet--just add the 2012 dates as necessary.
I've got a large and complex (80 tab) Excel file with various dates all over it, dating from 2003 to 2012.
In a few cases, I have made a new tab for the new year, but generally I don't. As GEWB suggested, formulas are easily transferred to an adjacent tab if needed.
Create a new tab (spreadsheet in the same file). Then poke the upperleftmost corner of the 2011 tab, to the left of column A and above row 1. That will select the entire sheet. Then right click and select copy. Then go to your newly created tab and select cell A1. Then paste.