Try this:

I'm assuming that cell A1 contains a true Excel date but is formatted as mmm
yyyy.

=DATE(YEAR(A1),MONTH(A1)-3,1)
=DATE(YEAR(A1),MONTH(A1)+2,1)

Format as mmm yyyy

Biff

"Eric" wrote in message
...
Does anyone know how to subtract/add periods in monthly base? for example,

Dec 2006 [in cell A1] - 3 [in cell B1] months = Sep 2006 [in cell C1]
Dec 2006 [in cell A1] + 2 [in cell B2] months = Feb 2007 [in cell C1]

and ignore to count the numebr of days, because the number of days for
each
month is different.
Does anyone have any suggestion?
Thank you
Eric