uptime

here's a quick way to tell how long your system has been running, using root, just issue:

#uptime

would show you more likely this output:
08:02:46 up 66 days, 58 min, 2 users, load average: 0.10, 0.11, 0.03

this is the same information of the first output line from issuing

#w

fields:
1 - current date and time
2 - uptime in days + hours + minutes
3 - number of currently logged in users
5.- your current load average stats

You can graph these number using tools like Cacti, MRTG or more.

See more with uptime.netcraft.com