"Matt's Dad" wrote...
....
=SUMPRODUCT(--(B$1=($B$8:$B$24)),--(DATE(YEAR(B$1),MONTH(B$1),1)<=($B$8:$B$24)),
--($A2=($A$8:$A$24)),$C$8:$C$24)
....

You could replace the first 2 terms with

--(TEXT(B$1,"YYYYMM")=TEXT($B$8:$B$24,"YYYYMM"))