I'm getting incorrect results when adding an interval in a DateTime input string.

Some strings work, some don't. Obviously adding zero shouldn't put the result 5 days in the future and truncate the time. Yikes. 

```
$dtTestStr='2018-09-05 10:25:00-04 + 0 year 0 mon 0 day 00:00:00';
$myFubar=new DateTime($dtTestStr);
echo $dtTestStr. ' is now '.$myFubar->format('Y-m-d H:i:s');

// '2018-09-05 10:25:00-04 + 0 year 0 mon 0 day 00:00:00 is now 2018-09-10 00:00:00'

```