uptime

uptime is a measure of the time a machine has up been.
This is pretty useful command.

To know since when the system is up one can do like this at the terminal prompt
$uptime
11:10:27 up 9 mins, 2 users, Load average: 0.02, 0.24, 0.18

Determining system uptime in windows -

1.

C:\> systeminfo | find "Time:"
System Up Time:            0 Days, 8 Hours, 7 Minutes, 19 Seconds

Note: Windows Vista Business 64-bit and Windows 7 do not return a "System Up Time" but "System Boot Time" instead.

2.

C:\> net statistics server | find "since"
Server Statistics for \\COMPUTERNAME
 
Statistics since 8/31/2009 8:52:29 PM