The uptime command shows the length of time since the system booted up. The output is displayed on the standard output.
break# uptime 2:49PM up 229 days, 30 mins, 2 users, load averages: 0.00, 0.00, 0.00
The above shows that, at 2:49pm (when the command was run), the system in question had been running for 239 days and 30 minutes. It also had two users logged on. The three load averages values show the system utilisation as at 1, 5 and 15 minutes ago respectively.
Note that some UNIX and Unix-like systems have a 'roll over' at 497 days where the counter it reset to zero. This means a system with 498 days of uptime will report 0 days and so on, for each 498th day.
See also w command.