Changing the cell locations of a forumla based on Date

Here's what I'm trying to do.

I have rows of data that have a date associated with them.

I want to be able to enter a date range in my spreadsheet and for a cell with a formula to determine based on those dates which cells to use in its calculations.

The way I do it now is to manually look at the data. I see that for example Feb 1st through Feb 15th is in cells B100 through B155. Then I manually change the formula to look at cells B100 through B155. The next day I have to change the formula again to look at 2/2 through 2/16 etc...

I can't cut and paste my spreadsheet, but I'll explain the issue better.

I have 2 columns. One has a date and another has a value. I want to be able to enter a date range. Let's say
01/02/2013 - 01/03/2013. I want a cell that will know to add the sum of B4 through B8 that correspond to those dates.The 4 and 8 in this case have to be a kind of variable value of the B column that will change depending which date
range I want.

A B

1 01/01/2013 10
2 01/01/2013 15
3 01/01/2013 17
4 01/02/2013 09
5 01/02/2013 05
6 01/02/2013 06
7 01/02/2013 15
8 01/03/2013 08
9 01/04/2013 20
1 01/04/2013 25

Thank you.

That did the trick. It worked so well, I may be back with another question.