Try:
=SUMPRODUCT((G2:G10=B2)*(G2:G10<=C2),K2:K10)

HTH
JG

"Tom" wrote:

I have a column "G" that contains several dates and another column "K" that
contain weights. Additionally, I have two cells (Start Date (B2) and End
Date "C2") that I would like to type in different dates which lookup in "G"
and sum the weights in "K".

G K
12/12/2005 200
12/16/2005 100
2/11/2005 300
11/15/2005 150

TFTH,
Tom