Uptime Script

I found a script that export the uptimes on computer to a csv. I modified it to suit my needs. I am new to powershell scripting and i wonder why do i get this error when running and what to do to get rid of it.

Thanks for helping

//// Error starts

Method invocation failed because [System.Management.Automation.PSObject] doesn't contain a method named 'op_Addition'.
At C:\Powerscipts\export_uptime2.ps1:29 char:19
+         $Results+= <<<< $Srv    
    + CategoryInfo          : InvalidOperation: (op_Addition:String) [], RuntimeException
    + FullyQualifiedErrorId : MethodNotFound

///

//Script starts

#add-PSSnapin quest.activeroles.admanagement
$Servers = Get-QADComputer -SearchRoot 'domain.local/workstations/std/Laptop' | foreach {$_.name}
$datetime = Get-Date -Format "yyyyMMddHHmmss";

#Add headers to log file
Add-Content "c:\temp\uptime-laptop.csv" "SystemName, Uptime";
foreach($Server in $Servers)

{
#$desc = Get-Qadcomputer $server | select Description
$wmi=Get-WmiObject -class Win32_OperatingSystem -ComputerName $Server -ErrorAction SilentlyContinue
if ($wmi)
        {
       
        $LBTime=$wmi.ConvertToDateTime($wmi.Lastbootuptime)
     
        [TimeSpan]$uptime=New-TimeSpan $LBTime $(get-date)

        $Srv = New-Object PSObject
        $Srv | Add-Member -MemberType NoteProperty -Name Computer -Value $Server
        $Srv | Add-Member -MemberType NoteProperty -Name Days -Value $Uptime.Days
        $Srv | Add-Member -MemberType NoteProperty -Name Hours -Value $Uptime.Hours
        $Srv | Add-Member -MemberType NoteProperty -Name Minutes -Value $Uptime.Minutes
        $Srv | Add-Member -MemberType NoteProperty -Name Seconds -Value $Uptime.Seconds
       
        $Results+=$Srv    
       
        }
}
$Results | Export-CSV "c:\temp\uptime-laptop.csv" -NoType