To get the uptime of a server in powershell run the following command:
Get-CimInstance -ClassName win32_operatingsystem | select csname, lastbootuptime
This works for Powershell v3 and v4. To run it against a remote server run the following code:
Get-CimInstance -ClassName win32_operatingsystem -ComputerName SERVERNAME| select csname, lastbootuptime
Please note that this only works if PS Remoting is enabled. To enable psremoting enter the command:
Enable-PSRemoting -Force
You can also type the command:
net statistics Workstation