Description:
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Checked on PHP 7.0.14-2+deb.sury.org~xenial+1, 7.0.8-0ubuntu0.16.04.3 and https://3v4l.org/

Note: This bug seems to effect all PHP versions but HHVM 3.13.2 isn't effected.

It seems that when creating a DateTime from a described string, the dates get handled incorrectly at some point. When using the values my test script it seems that the DateTimes are seen to be correct (when var_dump'd the values are correct) but when you call a method (in my test I'm calling diff but getTimestamp() also caused the issue in some way) the diff results are incorrect and the next var_dump shows the DateTime has been incorrectly adjusted.

In my case it seems that the diff is wrong because the DateTime object changes when calling diff(). My dates should be first of December to the 31st but the second var_dump will show the end date being the first of December as well.

If I partly describe the date I want in the string and use the modify() method to adjust by the "- 1 second" everything seems to work as expected. This implies that the "- 1 second" contributes to this bug but it's not the actual cause, something else must be going on internally.

Note: I did search for other issues and there are DateTime/diff related issues but none seems to exactly match this issue. They could be related but I just wanted to tell you what I've been able to find out and test.

Test script:
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https://gist.github.com/anonymous/4cdca5e99eb0e1003e04dc4b54c8958f

Expected result:
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date_test.php:11:int 30
date_test.php:16:int 30
date_test.php:24:int 30
date_test.php:29:int 30
date_test.php:37:int 30
date_test.php:43:int 30


Actual result:
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date_test.php:11:int 0
date_test.php:16:int 30
date_test.php:24:int 0
date_test.php:29:int 30
date_test.php:37:int 30
date_test.php:43:int 30