function Get-DateRFC2822() {
    [string]$date_string = ([System.DateTime]::Now | Get-Date).ToString("ddd, dd MMM yyyy HH:mm:ss zzz", ([System.Globalization.CultureInfo]::InvariantCulture))
    return $date_string
}

Running the command Get-DateRFC2822 returns the value

Sun, 06 Jan 2019 20:26:27 -0000