How long your computer has been running for - (the length of time between when your computer was turned on, and now.)

On a Linux/BSD system, you can get your uptime by using the uptime(1) command.
On a Microsoft Windows system, there is no easy way to see the uptime, but you can download the useful PSInfo utility, which will tell you this and more.

Windows systems tend to have smaller uptimes than Linux systems because you need to reboot Windows to apply security fixes. This doesn't necessarily imply the services are available for less time - given a cluster of machines, you won't reboot them all at the same time!