使用PHP发送电子邮件时，将电子邮件标题的日期更改为将来还是过去？

Question

我正在尝试编写一个脚本，该脚本可让我在任何给定时间发送电子邮件。 例如，我要发送日期为2018年的电子邮件。 这可能吗？

<?php
session_start();
$f_Submit = filter_var($_POST['submit'],FILTER_SANITIZE_STRING);
if ($f_Submit == 'Send')
{
if      (strcmp(md5(filter_var($_POST['user_code'],FILTER_SANITIZE_STRING)),$_SESSION['ckey']))
{ 

$header = "Location: ".basename ( __FILE__,"php")."php?msg=ERROR:   Invalid Verification Code";
header($header);
exit();
}
//------------------ Sanitizin the Input Variables   ------------------//
$f_fromname   = filter_var($_POST['fromname'],FILTER_SANITIZE_STRING);
$f_fromemail  = filter_var($_POST['fromemail'],FILTER_SANITIZE_EMAIL);
$f_reply      = filter_var($_POST['reply'],FILTER_SANITIZE_EMAIL);
$f_toemail    = filter_var($_POST['toemail'],FILTER_SANITIZE_EMAIL);
$f_bccemail   = filter_var($_POST['bccemail'],FILTER_SANITIZE_EMAIL);
$f_subject    = filter_var($_POST['subject'],FILTER_SANITIZE_STRING);
$f_message    = filter_var($_POST['message'],FILTER_SANITIZE_STRING);
//-------------------------------------------------------------------//
$date = new DateTime('2000-01-01');
$result = $date->format('D, d M Y H:i:s O');


if($f_reply=="") {
    $f_reply = $f_fromemail;
}

// Header
$headers = 'From:'.$f_fromname.' '.'<'.$f_fromemail.'>'. "\r\n";
$headers .= "Date: ".$result." \r\n";
$headers .= 'Bcc: '.$f_bccemail. "\r\n";
$headers .= 'Cc: ' . "\r\n";    
$headers .= "MIME-Version: 1.0\r\n";

?>

$ result变量不起作用。如果我发送邮件，它将显示发送邮件的日期，而不是我尝试设置的日期。