Summary: Use Windows PowerShell to find your system uptime.

 How can I use Windows PowerShell to easily find how long my computer has been running?

 Use the Get-Date cmdlet to return the current date and time, and then subtract the value
           of the LastBootUpTime property that comes from the Win32_OperatingSystem, for example:

(get-date) – (gcim Win32_OperatingSystem).LastBootUpTime

Note  GCIM is an alias for Get-CimInstance.