If you are reading in a standard RFC [2]822 date ("Sat, 07 Sep 2002 09:42:31 CST" format), you should be able to convert that to a PHP UNIX time integer via [man]strtotime/man, e.g.:
$timeUnixSeconds = strtotime($rssDateTimeValue);
If your calendar function just wants that PHP time value, you're all set. If it's looking for a date string of some sort, you can use the [man]gmdate/man function to output the value as desired, e.g.:
$dateTimeGmt = gmdate('r', $timeUnixSeconds);