uptime is a traditional Unix utility that shows the length of time since the system was booted. Some systems also display the load average (number of processes wanting to run averaged over 3 time intervals).

It is not distributed with Windows. A version has sometimes been available from Microsoft for NT4 and Windows-2000. The current version is http://support.microsoft.com/kb/232243 . It isn't released for systems after Windows-2000.

Some alternatives -:

The windows net utility shows a figure, but is not always correct. See my wrapper script.

Services for Unix includes an emulation of the Unix utmp/wtmp files. "uptime command for Services For Unix (Interix)" searches these files for the last startup record. Strictly this will be when the Interix subsystem started rather than the OS itself, so will be delayed for a few seconds. (Source uptime.c)