Usando Relative Formats para definir o período do DateInterval para alterar nossas datas

using_createfromdatedtring.php

<?php

$dateTime = new DateTime('2016-12-01 00:00:01');
echo $dateTime->format('d/m/Y H:i:s'), PHP_EOL;

//P1YT10H55S
$dateTime->add(DateInterval::createFromDateString('1 year + 10 hours + 55 seconds'));
echo $dateTime->format('d/m/Y H:i:s'), PHP_EOL;

//PT5H30M20S
$dateTime->sub(DateInterval::createFromDateString('5 hours + 30 minutes + 20 seconds'));
echo $dateTime->format('d/m/Y H:i:s'), PHP_EOL;

//P1Y4M10DT2H30M50S
$dateTime->add(DateInterval::createFromDateString('1 year + 4 months + 10 days + 2 hours + 30 minutes + 50 seconds'));
echo $dateTime->format('d/m/Y H:i:s');