There are two examples of command that I have found to be useful in determining the server uptime:
systeminfo | find "System Boot Time:"
net statistics server
The main difference I have found between the two is that the first option will give you actual time since last actual boot/power save state and the second option gives you “availability” – in the event you have a server that sleeps with WOL enabled it will give you the sum of the uptime plus sleeping states.
Microsoft used to provide a tool called uptime.exe for better management of time statistics, however it looks as though the project has been taken over by someone over at codeplex.com