Compare To Date Ranges?

I am developing a billing application to help out the billing lady who has been doing our billing by hand. I am essentially done�. Except for one class of customer I can�t wrap my head around. Nuts and Bolts:

1.Data comes from a table that shows Storage id, customer id, Billing Start date, Billing end date.

2.I am kicking the data back out to a table and I�ll build her a nice little crystal she can run.

Problem:We rent storage space. If multiple customers use the same storage then they will split the cost of the daily fee. If there is 3 customers they each pay a third etc. This seems simple but If Customer 1 starts on 1/1/12 and ends 1/10/12 she pays 10 days. If Customer 2 comes in 1/3/12-1/9/12 they will split 7 days. If Customer 3 comes in only on 1/9/12 then they will all pay a third on that day. Now imagine this across 600+ storage units and 1000+ customers and each customer using multiple storage at any given time.

My biggest issue is I don�t know how to express how we do that calculation by hand programmatically. By hand process and how I figure I am eating the elephant: Find every Customer that has used the Space in the billing period -Determine the unique storage numbers that were used during the billing cycle kick the results to a storage table on an access database. I am using a table because I figured it would be less of burden for the application then a list or array.

Determine where there is overlap - Use �FOR� to cycle through the orginal data table using the unigue. Here is where I am stuck. How do I compare the dates in a way to determine which customer is using it when compared to the others. Obviously i can determine the difference in dates between the start and end. It is getting the applicaiton to recognize the dates are with in the other dates. here is kind of what I was thinking obviosly this doesn't work I am just using the format so don't crush me too bad because I code like a caveman.

Dim CustomerDifference as time span
Dim UniqueStorage as Integer
Dim Customer as string

[code]....