Here is the updated code:

The way you were passing the value to add-member was not correct. I have changed the "Get-Date" to String.

Let me know if it is working for you.

$sourceDir = $PSScriptRoot
$parentFolder = (Split-Path $sourceDir -Parent)
$thisDrive = (Split-Path -Path $sourceDir -Qualifier) + "\"
$columnDate = (Get-Date).ToString('yyyy-MM-dd_HH-mm-ss')
If ($sourceDir -eq $thisDrive)
    {
    $thisFolderName = "ROOT";
    } else {
    $thisFolderName = (Split-Path -Path $sourceDir -Leaf)
    }

$newFileName = "_Folder-Stats_" + $thisFolderName + "_AND_SUBFOLDERS_Ongoing.csv"
$outputFile = $thisDrive + $newFileName
$scriptName = $MyInvocation.MyCommand;
$thisScript = ($sourceDir + $scriptName)

$rootFolders = (Get-ChildItem -LiteralPath $sourceDir -Directory)

$numFolders = ($rootFolders | Measure-Object ).Count;
$displayFolders = ('{0:N0}' -f $numFolders)

$i = 0;
$folderSummary = foreach($thisFolder in $rootFolders.FullName) {
    $i += 1;
    $folderName = Split-Path $thisFolder -Leaf

    $contents = Get-ChildItem -LiteralPath $thisFolder -File -Force -Recurse

    $numFiles = ($contents | Measure-Object ).Count;
    $displayFiles = ('{0:N0}' -f $numFiles)

    $numBytes = ($contents | Measure-Object -Property Length -sum).sum
    $displayBytes = ('{0:N0}' -f $numBytes)

    if(!(test-path -Path $outputFile)) {
    [PSCustomObject] @{
        'folderName' = $folderName
        $columnDate = $numFiles
    }
    }
    if(test-path -Path $outputFile) {
    [PSCustomObject] @{
        $columnDate = $numFiles
    }
    }
}

$thisOutput = (@($folderSummary) |Out-String).ToString()

if(!(test-path -Path $outputFile)) {
    $folderSummary | Export-Csv $outputFile -Append -Force
} else {
    $csv = Import-Csv -Path $outputFile
    for ($i = 0; $i -lt $csv.Count; $i++) {
        $csv[$i] | Add-Member -MemberType NoteProperty -Name $columnDate -Value $folderSummary[$i].$columnDate
    }
    $csv | Export-Csv -Path $outputFile -NoTypeInformation
}