Description:
------------
I had a test begin to fail.  I was naively expecting subtraction of an interval 'P18Y' to yield a date on which all users born that day would turn 18 years old today.  Although the leap day should be an exception for my test case (https://www.quora.com/If-youre-born-on-February-29th-when-do-you-legally-turn-one-year-older) I noticed that there appears to be some brokenness with subtraction of 'P1D' interval from the leap day, and the subsequent subtraction of the 'P18Y' interval from that result.

p.s. My Turing test asks for the solution to 50 - 32 -- spooky :-)

Test script:
---------------
<?php
/* get some significant dates */
$today = new DateTime();
echo("\nTODAY\n");
var_export($today);
$minorAgeInterval = new DateInterval('P18Y');
$oneDayInterval = new DateInterval('P1D');
$tomorrow = $today->add($oneDayInterval);
echo("\nTOMORROW\n");
var_export($tomorrow);
$yesterday = $today->sub($oneDayInterval);
echo("\nYESTERDAY\n");
var_export($yesterday);
$todayMinus18 = $today->sub($minorAgeInterval);
echo("\nTODAY MINUS 18\n");
var_export($todayMinus18);
$tomorrowMinus18 = $tomorrow->sub($minorAgeInterval);
echo("\nTOMORROW MINUS 18\n");
var_export($tomorrowMinus18);
$yesterdayMinus18 = $yesterday->sub($minorAgeInterval);
echo("\nYESTERDAY MINUS 18\n");
var_export($yesterdayMinus18);
?>


Expected result:
----------------
paul@paul-VirtualBox:~$ php php_date_fun.php 

TODAY
DateTime::__set_state(array(
   'date' => '2016-02-29 15:49:56',
   'timezone_type' => 3,
   'timezone' => 'America/Los_Angeles',
))
TOMORROW
DateTime::__set_state(array(
   'date' => '2016-03-01 15:49:56',
   'timezone_type' => 3,
   'timezone' => 'America/Los_Angeles',
))
YESTERDAY
DateTime::__set_state(array(
   'date' => '2016-02-28 15:49:56',
   'timezone_type' => 3,
   'timezone' => 'America/Los_Angeles',
))
TODAY MINUS 18
DateTime::__set_state(array(
   'date' => '1998-03-01 15:49:56',
   'timezone_type' => 3,
   'timezone' => 'America/Los_Angeles',
))
TOMORROW MINUS 18
DateTime::__set_state(array(
   'date' => '1980-03-01 15:49:56',
   'timezone_type' => 3,
   'timezone' => 'America/Los_Angeles',
))
YESTERDAY MINUS 18
DateTime::__set_state(array(
   'date' => '1998-02-28 15:49:56',
   'timezone_type' => 3,
   'timezone' => 'America/Los_Angeles',
))

Actual result:
--------------
paul@paul-VirtualBox:~$ php php_date_fun.php 

TODAY
DateTime::__set_state(array(
   'date' => '2016-02-29 15:49:56',
   'timezone_type' => 3,
   'timezone' => 'America/Los_Angeles',
))
TOMORROW
DateTime::__set_state(array(
   'date' => '2016-03-01 15:49:56',
   'timezone_type' => 3,
   'timezone' => 'America/Los_Angeles',
))
YESTERDAY
DateTime::__set_state(array(
   'date' => '2016-02-29 15:49:56',
   'timezone_type' => 3,
   'timezone' => 'America/Los_Angeles',
))
TODAY MINUS 18
DateTime::__set_state(array(
   'date' => '1998-03-01 15:49:56',
   'timezone_type' => 3,
   'timezone' => 'America/Los_Angeles',
))
TOMORROW MINUS 18
DateTime::__set_state(array(
   'date' => '1980-03-01 15:49:56',
   'timezone_type' => 3,
   'timezone' => 'America/Los_Angeles',
))
YESTERDAY MINUS 18
DateTime::__set_state(array(
   'date' => '1962-03-01 15:49:56',
   'timezone_type' => 3,
   'timezone' => 'America/Los_Angeles',
))