ISO code example
Example: iso-8601
echo date('c'); //2021-02-03T21:42:51-08:00
echo date(DATE_ISO8601); //2021-02-03T21:42:51-0800
$datetime = new DateTime();
echo $datetime->format(DateTime::ATOM); //2021-02-03T21:42:51-08:00
echo date('c'); //2021-02-03T21:42:51-08:00
echo date(DATE_ISO8601); //2021-02-03T21:42:51-0800
$datetime = new DateTime();
echo $datetime->format(DateTime::ATOM); //2021-02-03T21:42:51-08:00