You get even more interesting result using datetime.

Code:
-------
<?php

$old = ['created' => new \DateTime()];
$new = ['created' => new \DateTime('+1 minute')];

$changeset = array_merge_recursive($old, $new);

var_dump($changeset);

Expected:
---------
array(1) {
  ["created"]=>
  array(2) {
    [0] => object(DateTime)#1 (3) {
      ["date"]=>
        string(26) "2015-11-04 10:36:03.000000"
      ["timezone_type"]=>
        int(3)
      ["timezone"]=>
        string(16) "Europe/Amsterdam"
    },
    [1] => object(DateTime)#1 (3) {
      ["date"]=>
        string(26) "2015-11-04 10:37:03.000000"
      ["timezone_type"]=>
        int(3)
      ["timezone"]=>
        string(16) "Europe/Amsterdam"
    }
  }
}

Actual
-------
array(1) {
  ["created"]=>
  array(3) {
    ["date"]=>
    array(2) {
      [0]=>
      string(26) "2015-11-04 10:36:03.000000"
      [1]=>
      string(26) "2015-11-04 10:37:03.000000"
    }
    ["timezone_type"]=>
    array(2) {
      [0]=>
      int(3)
      [1]=>
      int(3)
    }
    ["timezone"]=>
    array(2) {
      [0]=>
      string(16) "Europe/Amsterdam"
      [1]=>
      string(16) "Europe/Amsterdam"
    }
  }
}

So that treats DateTime as array.

I have to admit that this behavior[1] is unintuitive, and might
not have been the best design decision.  However, changing it
after so many years is likely to break code which relies on it. As
such, this should go through the RFC process[2].  Anybody is
welcome to pursue it.

Anyhow, more important than to change the behavior, is to properly
document it.  There is no indication on the man page[3], so I'm
changing this ticket to documentation problem.

[1] <https://3v4l.org/vmrnK>
[2] <https://wiki.php.net/rfc/howto>
[3] <https://www.php.net/array_merge_recursive>