<?php

public function testInterval()
{
    $date = new DateTime('+1 month');

    $date->add(\DateInterval::createFromDateString('3 months'));

    // Tested on 2016-08-31
    // Failed asserting that two DateTime objects are equal.
    // Expected :2016-12-31T14:28:49+0000
    // Actual   :2017-01-01T14:28:49+0000
    static::assertEquals(new DateTime('+4 months'), $date);
}