I2=IFERROR(SUMPRODUCT(--(TEXT($B$2:$B$1001,"mmyy")=TEXT(G2,"mmyy")),$D$2:$D$1001)/SUMPRODUCT(--(TEXT($B$2:$B$1001,"mmyy")=TEXT(G2,"mmyy")),--ISNUMBER($D$2:$D$1001)),"")
Try this It will rectify your problem
you have text in Column D, so that you got the wrong result
the formula in Column D =IF(ISNUMBER(C2-B2),C2-B2,"") giving text "" in column D