Convert a localized date from an html5 input[type=date] into an ISO8601 datetime string

storedate.php

<?php
  $dateParam = $this->request->getRequiredBodyParam('date');
  $timezone = new \DateTimeZone($dateParam['timezone']);
  $date = new \DateTime($dateParam['date'], $timezone);

  // don't know why, but the date needs the offset
  // otherwise it gets stored as the previous day
  $date->add(new \DateInterval('PT'.$date->getOffset().'S'));

  $element->uid = StringHelper::UUID();
  $element->date = $date->format(\DateTime::ISO8601);
?>