It is required to perform a dummy update to the cell before Excel will compute/display correct result. The problem was oberved with MS Excel 2002 and 2007, although each version had unique reactions. Open Office, on the other hand, has no problem displaying the correct result right from the start, without intervention. Thought the problem might be the entire column range, so tried variations: in the write_formula call: =SUMPRODUCT((DATA!K:K>=DATEVALUE("1/01/2007"))*(DATA!K:K<DATEVALUE("2/01/2007"))*(LEFT(DATA!D:D,3)="3.1")) =SUMPRODUCT((DATA!K2:K200>=DATEVALUE("3/01/2007"))*(DATA!K2:K200<DATEVALUE("4/01/2007"))*(LEFT(DATA!D:D,3)="3.0")) =SUMPRODUCT((DATA!K2:K200>=DATEVALUE("1/01/2007"))*(DATA!K2:K200<DATEVALUE("2/01/2007"))*(LEFT(DATA!D2:D200,3)="3.0"))