In Excel 2003 that construct is explicitly allowed. The help for the DATE
function includes:
"Month is a number representing the month of the year. If month is
greater than 12, month adds that number of months to the first month in the
year specified. For example, DATE(2008,14,2) returns the serial number
representing February 2, 2009."

I would be very doubtful whether Excel 2007 has changed this, and your two
other sheets suggest not, so my first thought would be to check whether your
faulty sheet has macros which are confusing the issue?
--
David Biddulph


"Brad" wrote in message
...
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?