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