Change Email Credit Monthly on our Email Marketing system

Current we use Interspire Email Marketer and modified it to suit our needs. We need a developer who can figure out a glitch that renews all accounts on monthly basis according to system time. We need it to calculate monthly timing for each account based on the created date for each user NOT the system time. I am sure its a small variable change. The created date for each account does show up on the admin panel. It just needs to be implemented so that the email credits will be renewed every month for each user account according to their own created date. If this job is nicely done, we will award our 3 upcoming projects to the same user too.