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