How do others define a business day metric?

I am trying to get an idea on how other organizations define their business day metrics. We have a central site where the service desk is located and operating 24x7 with several outlying sites operating 12x5 consisting of 4 different time zones. When a customer calls in for a service request, how do you measure the performance of their ticket? We have defined a business day as 0600 to 1800 M-F. But we are trying to decide how to apply this to the customers expectations. Do you base the business day upon where they are located even if the support being provided might be in a different time zone? If you base it on the businesses location, what happens if the location of support changes to a new timezone? Does the metric start on the day they call, or the following full business day? I have tried looking for how other businesses define certain metrics or sla's, but have come up with very little.

If you ask 5 people, you will probably get 5 different answers. This is something you really need to discuss with your business people and decide what is important to them. From a design standpoint, you most certainly want to track things by the customer's time, the support group's time and universal time. And it would seem reasonable that there are different metrics from different points of view: the customer's against SLAs and internally against expectations.