Cond Format Calculation Returns Different Values On Different Computers

Greetings -

I have the following calculation in conditional formatting for the purpose of highlighting the most recently completed week number.

=(WEEKNUM(TODAY(),1))+12

On one computer, running 2010 Excel in Windows XP, this highlights Week 16.

On another computer, running 2007 Excel in Windows 7, this highlights Week 17.

Any ideas as to why this is happening and what can be done about would be greatly appreciated.

Thanks,

Aaron

=TODAY() returns the same date on both computers...

- Aaron