change_timezone_in_iso8601_date_string.php

<?php
  // if it's not a ISO8601 date string, skip it
  if(stripos($v , "T") === false) continue;

  $dateval = $v;

  // create ISO8601 dateTime
  $date = DateTime::createFromFormat(DateTime::ISO8601, $dateval);
  // set to user's timezone
  $date -> setTimeZone($dt_user_timezone);
  // assign back to main events array
  $events[$id][$k] = $date -> format(DateTime::ISO8601);