Using a conditional function to sum the difference between two columns.

Column C has Issue dates
Column D has Due dates (ALWAYS later than Issue Dates C)

Task is to sum the difference between columns D and C but skip those rows
that have blank in column D.

I attempted to use €œIF€� and €œSUMPRODUCT€� in the following formula to add the
different between two column skipping those €œD€� cells that were blank. The
data in the cells are dates, however I also tried it with just plain whole
numbers and had a similar result.

If I use the formula on a single cell where the €œD€� cell is blank, i.e. the
range is €œD2:D2-C2:C2€� it will show a result of blank. Also it will work
when I expand the range until it includes a blank cell, then it subtracts the
negative value of the €œC€� cell from the total. The formula doesnt seem to
be skipping the rows with blank cells but rather subtracting the value of €œC€�.

The result below should be 87, however it comes back as -39397

€¦and use I used the Shift-Ctrl-Enter

{=IF(A$1:A60,SUMPRODUCT(B$1:B6-A$1:A6),"")}

A B
1 02/06/08 02/15/08
2 02/06/08 02/15/08
3 02/06/08 02/15/08
4 02/06/08
5 02/06/08 03/31/08
6 02/14/08 02/20/08

Thank you for your help.