gistfile1.php

 <?php
 // php wtf of the day
 
 use \DateTime,
     \DateInterval;
 
 $now = ['ref'=> new DateTime(), 0 => new DateTime(), 1 => new DateTime];
 $interval = new DateInterval('PT10S');
 
 // Have to do this to make it work. Just putting it here, so we can analyse the difference
 $now[1]->sub($interval)->sub($interval);
 $tries = [$now[0]->diff($now[0]->sub($interval)->sub($interval), true), //not working...
           $now['ref']->diff($now[1], true)]; // working
 
 var_export($tries);

gistfile2.php

<?php
array (
  0 => 
  DateInterval::__set_state(array(
     'y' => 0,
     'm' => 0,
     'd' => 0,
     'h' => 0,
     'i' => 0,
     's' => 0,
     'invert' => 0,
     'days' => 0,
  )),
  1 => 
  DateInterval::__set_state(array(
     'y' => 0,
     'm' => 0,
     'd' => 0,
     'h' => 0,
     'i' => 0,
     's' => 20,
     'invert' => 0,
     'days' => 0,
  )),
)%