Out-File Question

I have this code that works on my local Windows 7 machine.

(get-wmiobject win32_operatingsystem).converttodatetime((get-wmiobject win32_operatingsystem).installdate)

but both sets of code below fail.  Both create the .csv file but nothing gets written to it.   Why?  

$PCs = Get-Content "c:\temp\tmp\machines.txt"
$OSDate = (get-wmiobject win32_operatingsystem).converttodatetime((get-wmiobject win32_operatingsystem).installdate)

Foreach ($machines in $PCs) {
    $OSDate | Out-File "c:\temp\tmp\OSInstallDate.csv"
    }

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$PCs = Get-Content "c:\temp\tmp\machines.txt"


Foreach ($machines in $PCs) {
    (get-wmiobject win32_operatingsystem).converttodatetime((get-wmiobject win32_operatingsystem).installdate) | Out-File "c:\temp\tmp\OSInstallDate.csv"
    }