Show Billing Period

Hi, I want help regarding showing the Billing Period for each of my customer
in the database.The Billing Period is Generally of 30 days for each
customer.I had many customers with different Billing Cycles for each of them.

My question to the forum,is How would i show the billing periods for each
customer basing on the first bill generated date.

For example, if A's bill is generated for the first time on 23/04/2010,with
the first billing period as 22/03/2010 - 22/04/2010.The Next billing cycle
is for 30 days.

How to calculate the Next billing cycle for each month for the same customer.

Please give me any ideas on working about this problem or provide me any
examples.

Thanks