how to get the application running time - iphone

I want to calculate the running time of application that is when the application was started and then when it was closed.

Starting can be logged 开发者_运维知识库in application did finish launching but application will terminate is not working for me as when home button is pressed the application fires event "applicationWillResignActive" but user may start the application again or user can quit the application. When I try to quit the application application will terminate event is not occurring.

How can I then find out the application running time ?

Many Thanks,

Naveed Butt

If you're on iOS 4, you probably want applicationDidEnterBackground