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The DatePart(ww,@Date) function also may misrepresent WeekNumber depending on your perspective.

1/1/2010 is a Friday and really the last "business week" of 2009 which starts on Sunday 12/27/2009 thru Sat 1/2/2010.

You may also need to ignore the fact that business work weeks often start on Monday thru Sunday, but that's another local altogether...

Other WorkWeek calculations can be local modifications to this tool.