I have 3 fixed dates that represent effective date of
prices:

01/01/2004
04/01/2004
07/01/2004
12/31/2099

Now, I have a billing period (variable) that falls
somewhere between 01/01/04 and 12/31/2099;
i.e. 3/15/2004 to 4/15/2004

I need to be able to prorate across these price changes
based upon the billing period, which is a variable.
Specifically, I need to know the number of days to charge
at 01/01/2004 prices, 04/01/04 prices, etc.

Any assistance in regards to how this can be done in
Excel would be appreciated.