I use this little PowerShell snippet:
function Get-SystemUptime {
$operatingSystem = Get-WmiObject Win32_OperatingSystem
"$((Get-Date) - ([Management.ManagementDateTimeConverter]::ToDateTime($operatingSystem.LastBootUpTime)))"
}
which then yields something like the following:
PS> Get-SystemUptime
6.20:40:40.2625526