The amount of time since the system was last booted

I have used both uptime and cat /proc/uptime commands. I know the idle time in the later can be greater than running time (multiprocessors). However, when I add both running and idle times, and compare it to the uptime (5:30 hrs) they are not equal. I assume it's because the uptime returns the real time and the cat /proc/uptime returns processor(s) time. Short things clear, I want to know the reason exactly and I want to know which one to choose, according to the topic. Also, is it possible to get the processor(s) time in the last **24 hours?** **NOTE:** Please read this article to understand where this is going. *Distributor ID: Ubuntu Description: Ubuntu 12.04.2 LTS Release: 12.04 Codename: precise *