Try using this array formula

=SUM(IF(List!B1:B23<=A1,List!C1:C23,0))

Instead of hitting Enter after typing in the formula, press
Ctrl-Shift-Enter simultaneously. That will result in the formula
having curly brackets around it like.

{=SUM(IF(List!B1:B23<=A1,List!C1:C23,0))}

The reason your other formula was not working was that the criteria
"<=A1" was not true. You can't use a cell reference in quotes. If you
had entered in the actual data that appeared in A1 sya a date.
"<=12/1/2005", it would have worked. I assume that the data in A1 will
change.


HTH

Cheers,

Steve


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