Rolling 30 days

hey guys,

I have been struggling to get this problem done:

There are 3 columns,
1st column: 1000 dates (for example: 1/02/05(taking this as Monday), 2/02/05(tuesday), 3/02/05, 4/02/05, 5/02,05, 8/02/05(monday), etc...) where weekends and public holidays are not included.
2nd column: 1000 values (for example: 20, 21, 26, 30, 10, 9 etc...)
3rd column: change (21-20 = 1, 26-21=5,...., 9-10=-1. etc...)

All i need to do is find the total change in for monthly moves (not calender months). for example, 15/06/05 - 15/07/05.
Since the weekends and public holidays are not included in the dates, i am really confused.. Need help getting the formula too

Hope someone could assist me with this.
Thank you so much!!