Description:
------------
Some dates falling on February 29th in negative years can not have an interval of 
one day added to them.

The pattern appears to be years of the form -n16, but some years behave as 
expected (e.g. -816 and -416 in the test script below).

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

for ($i = -1000; $i < 1000; $i++) {
    $date = new DateTime('midnight', new DateTimeZone('UTC'));
    $date->setDate($i, 2, 29);

    // Rolled over into March, no Feb 29th, skip
    if ($date->format('M') !== 'Feb') {
        continue;
    }

    $next = clone $date;
    $next->add(new DateInterval('P1D'));

    if ($date->format('Y-m-d') === $next->format('Y-m-d')) {
        echo $next->format('Y-m-d l') . PHP_EOL;
    }
}

?>

Expected result:
----------------
No output is expected.

Actual result:
--------------
-0916-02-29 Friday
-0716-02-29 Tuesday
-0616-02-29 Sunday
-0516-02-29 Friday
-0316-02-29 Tuesday
-0216-02-29 Sunday
-0116-02-29 Friday