[otrs] calculating answer time

Hi, I sent this a few days ago, but unfortunately haven't received a response. Maybe because it's not possible? We'd like to be able to calculate the average time it takes for us to answer a ticket. I found code in the logs, but it has one problem, best illustrated through an example: If a customer writes to us on Nov 1 and we answer immediately, then he writes again on November 5 and we answer immediately, the "time difference" calculated is 4 days. This messes up our statistics - since really we answered immediately each time! Is there any way to solve this? It's very important for us! Thanks very much and happy holidays! Tabitha