monthly calculation for storage
 

am trying for long time to make formulas which would calculate storage period on monthly basis. Conditions for storage a 42 days free, than 43-60 1st tariff apply, than 61-90 2nd tariff and after 90 days 3rd tariff apply. Calculation should be for goods left w/h in that particular month and also for the goods which will stay on stock.

Example:
IN QTY KGS IN DATE END FREE TIME OUT QTY KGS OUT DATE
10.000 01.08.11 12.09.11 5.000 10.01.12
10.000 01.11.11 13.12.11 5.000 01.02.12
10.000 15.12.11 26.01.12 5.000 10.02.12
10.000 30.12.11 10.02.12 0 -

How to calculate storage for January 2012 ?

Thanks in advance for assistance