Q. Can I use System Idle time to check how long my box has been running?

A. Sadly, no. The System Idle time reported in Task Manager is essentially the amount of time the CPU has been idle. In a busy box, this counter wouldn't be accurate, but it's even worse than that in a multi-core machine. Each core's time is considered, so if I have four idle cores, the System Idle time would increase four seconds each second. (Make sure you enable Show processes from all users if you can't see your System Idle time in Task Manager.)

In the picture below, you can see Task Manager on my 16-core server (well, eight cores with Hyper-Threading). Task Manager shows a much longer System Idle time than my system really has—every second the System Idle time increases by 16 seconds (when the server is idle). As you can see, System Idle shows the server as being up for 53 days (1272 hours), but in reality it had only been three days as of when I wrote this on Feb. 23.

Click to expand.