Private/Update-specTaskXmlStartBoundary.ps1

function Update-specTaskXmlStartBoundary {
    [CmdletBinding()]
    param(
        [Parameter(Mandatory)]
        [xml]$TaskXml,

        [Parameter(Mandatory)]
        [datetime]$NewTime
    )

    $triggers = $TaskXml.Task.Triggers.ChildNodes | Where-Object { $_.GetType().Name -eq 'XmlElement' }
    foreach ($trigger in $triggers) {
        if ($trigger.StartBoundary) {
            $originalDate = [datetime]$trigger.StartBoundary
            $newBoundary = '{0:yyyy-MM-dd}T{1:HH:mm:ss}' -f $originalDate, $NewTime
            $trigger.StartBoundary = $newBoundary
        }
    }
    return $TaskXml
}