With defined names and without DATEDIF
Excel 2007
Start 01/01/08 (Y2008)
End 12/31/08 (Y2008)
Begin 01/15/08
Finish 06/26/08
D08D
=MONTH(ROW(INDEX($A:$A,Begin):INDEX($A:$A,Finish)) )
Y08D
=MONTH(ROW(INDEX($A:$A,Start):INDEX($A:$A,End)))
MoSeq
=ROW(INDEX($A:$A,MONTH(Start)):INDEX($A:$A,MONTH(E nd)))
Count of whole months
=SUMPRODUCT(--(FREQUENCY(Y08D,MoSeq)=FREQUENCY(D08D,MoSeq)))-1
=4