//Object Oriented Method
$datetime = new DateTime('2010-12-30 23:21:46');
echo $datetime->format(DateTime::ATOM); // Updated ISO8601
//Procedural Method
echo date(DATE_ISO8601, strtotime('2010-12-30 23:21:46'));(new DateTime('2022-05-01T16:00:00-03:00'))->format('Y-m-d\TH:i:sp');$startTime = new DateTime($appointment_date . ' ' . $booking_slot[0]); // 2021-12-28 0900
$startTime = $startTime->format('Y-m-d\TH:i:sp'); // 2021-12-25T12:02:00Z
FYI: there's a list of constants with predefined formats on the DateTime object, for example instead of outputting ISO 8601 dates with:
<?php
echo date('c');
?>
or
<?php
echo date('Y-m-d\TH:i:sO');
?>
You can use
<?php
echo date(DateTime::ISO8601);
?>
instead, which is much easier to read.