You're the first person I've helped with this so far, but that date is indeed controlled by the script. In your mail_PHP.php file, find something that looks like this:
$mailHeader .= "Date: ". date('r (T)').$lineEnd;
Change the line to look like this:
$mailHeader .= "Date: ". date('m-d-Y G:i:s (T)').$lineEnd;
Or you could change the format to look like how you want it to look using this guide:
function.date.php