Something similar to linux
cat /proc/uptime
which returns the uptime in seconds, and preferably not parsing uptime(1).
The Uptime article on Wikipedia has an interesting lead:
Using sysctl
There is also a method of using sysctl to call the system's last boot time: $ sysctl kern.boottime kern.boottime: { sec = 1271934886, usec = 667779 } Thu Apr 22 12:14:46 2010