Fejl med DateTime

fejl:
Fatal error: Cannot redeclare class DateTime in D:\Domains\moedested.dk\wwwroot\lib\lib.datetime.php on line 13

class DateTime
Line 13-->    {
        # Returns 3 drop-down boxes. Day, Month, Year.
        function GetDOBSelector ($dob = 0)
        {
            $months = array(1 => "January", "February", "March", "April", "May", "June", "July", "August", "September", "October", "November", "December");

            if ($dob)
            {
                $_POST["dob_day"] = date("d", $dob);
                $_POST["dob_month"] = date("m", $dob);
                $_POST["dob_year"] = date("Y", $dob);
            }