How to calculate No. of Days Credit

Dear Esteemed Member,
could anyone guide me that how to calculate the No. of Credit Days availed by Customer.(Debtor).
Like Someone gives credit of 30 Days interest free for each delivery. customer alwasy take daily delivery. now customer doest not make any payment.
how to calculate credit days availed by customer .
any method like Average or else with example without Example.

please guide.