Select value from row above/after

Hi,

I have a table reports like the following

Date ID Sum
10/3/2010 1 6
11/3/2010 1 5
12/3/2010 1 8
16/3/2010 1 5
20/3/2010 1 12

The user would select a specific date and would like to know the value in the sum field several rows before (it could be any number of rows before).
The dates are not consecutive, as seen in the table.

for example. I can select 16/3/2010 and 2 rows before. The return value would be 5, as this is the sum column value at date 11/3/2010.

Any ideas?

Thanks,
T.