strComputer = "."
Set objWMIService = GetObject("winmgmts:{impersonationLevel=impersonate}!\\" & strComputer & "\root\cimv2")

Set objOS = objWMIService.ExecQuery("Select * from Win32_OperatingSystem")
 
For Each strOS in objOS
    dtmInstallDate = strOS.InstallDate
    strReturn = WMIDateStringToDate(dtmInstallDate)
    Wscript.Echo strReturn
Next
 
Function WMIDateStringToDate(dtmInstallDate)
    WMIDateStringToDate = CDate(Mid(dtmInstallDate, 5, 2) & "/" & Mid(dtmInstallDate, 7, 2) & "/" & Left(dtmInstallDate, 4) & " " & Mid (dtmInstallDate, 9, 2) & ":" & Mid(dtmInstallDate, 11, 2) & ":" & Mid(dtmInstallDate, 13, 2))
End Function