Description:
------------
timelib_time->timelib_rel_time is mutated by date_modify, and because it is never zero'ed out it latters causes other date operations to fail (like date_date_set)

A reproduce is attached, and I will open a PR with a possible patch. 

Test script:
---------------
<?php

$date = new DateTime('2011-12-25 00:00:00');
$date->modify('first day of next month');
$date->setDate('2012', '1', '29');
var_dump($date);
----

https://3v4l.org/PASOu

Expected result:
----------------
object(DateTime)#1 (3) {
  ["date"]=>
  string(26) "2012-01-29 00:00:00.000000"
  ["timezone_type"]=>
  int(3)
  ["timezone"]=>
  string(16) "Europe/Amsterdam"
}

Actual result:
--------------
object(DateTime)#1 (3) {
  ["date"]=>
  string(26) "2012-01-01 00:00:00.000000"
  ["timezone_type"]=>
  int(3)
  ["timezone"]=>
  string(16) "Europe/Amsterdam"
}