The uptime command prints the current time, the length of time the system has been up, and the average number of jobs in the run queue over the last 1, 5 and 15 minutes. It is, essentially, the first line of a w(1) command.
Below is an example of the output uptime provides:
example% uptime 10:47am up 27 day(s), 50 mins, 1 user, load average: 0.18, 0.26, 0.20
See attributes(5) for descriptions of the following attributes:
|ATTRIBUTE TYPE||ATTRIBUTE VALUE|
who -b gives the time the system was last booted.