Excel 2007

On one sheet
=date(2008,13,1) provides the correct answer of 1/1/2009

copy that equation to a different sheet (in the same workbook) provides the
incorrect answer of #NUM!

copy that equation to a third sheet (in the same workbook) provides the
correct answer of 1/1/2009

Can't figure why I'm getting the #NUM!. Any suggestions?