The difference for 8/31/08 will be :
=IF(ISNA(MATCH(D1,B:B,0)),D1-VLOOKUP(D1,B:B,1,1),D1)
HTH
Daniel

Is there a way with a given date that it knows which date range or period
that it belongs to? If I have a date of 8/31/08, I want find which range it
is in between. Therefore, it is after 08/11/08 and before 09/07/08. Once I
find it, I want to compute the 8/31/08 minus 08/11/08 then multiple the
volume times the given rate for that range. Does this make sense? I
appreciate any help. Thanks.
Period 1 12/31/07 01/27/08
Period 2 01/28/08 02/24/08
Period 3 02/25/08 03/23/08
Period 4 03/24/08 04/20/08
Period 5 04/21/08 05/18/08
Period 6 05/19/08 06/15/08
Period 7 06/16/08 07/13/08
Period 8 07/14/08 08/10/08
Period 9 08/11/08 09/07/08
Period 10 09/08/08 10/05/08
Period 11 10/06/08 11/02/08
Period 12 11/03/08 11/30/08
Period 13 12/01/08 12/28/08