Description:
------------
When changing the time zone from UTC to -09:00 the local date gets updated but after changing it back to UTC it doesn't.

Both DateTime and DateTimeImmutable have the same issue.

Test script:
---------------
$date = new DateTime('2015-09-01+00:00');
var_dump($date);

$date = $date->setTimezone(new DateTimeZone('-09:00'));
var_dump($date);

$date = $date->setTimezone(new DateTimeZone('+00:00'));
var_dump($date);

$date = new DateTimeImmutable('2015-09-01+00:00');
var_dump($date);

$date = $date->setTimezone(new DateTimeZone('-09:00'));
var_dump($date);

$date = $date->setTimezone(new DateTimeZone('+00:00'));
var_dump($date);

Expected result:
----------------
object(DateTime)#1 (3) {
  ["date"]=> string(26) "2015-09-01 00:00:00.000000"
  ["timezone_type"]=> int(1)
  ["timezone"]=> string(6) "+00:00"
}
object(DateTime)#1 (3) {
  ["date"]=> string(26) "2015-08-31 15:00:00.000000"
  ["timezone_type"]=> int(1)
  ["timezone"]=> string(6) "-09:00"
}
object(DateTime)#1 (3) {
  ["date"]=> string(26) "2015-09-01 00:00:00.000000"
  ["timezone_type"]=> int(1)
  ["timezone"]=> string(6) "+00:00"
}
object(DateTimeImmutable)#2 (3) {
  ["date"]=> string(26) "2015-09-01 00:00:00.000000"
  ["timezone_type"]=> int(1)
  ["timezone"]=> string(6) "+00:00"
}
object(DateTimeImmutable)#3 (3) {
  ["date"]=> string(26) "2015-08-31 15:00:00.000000"
  ["timezone_type"]=> int(1)
  ["timezone"]=> string(6) "-09:00"
}
object(DateTimeImmutable)#1 (3) {
  ["date"]=> string(26) "2015-09-01 00:00:00.000000"
  ["timezone_type"]=> int(1)
  ["timezone"]=> string(6) "+00:00"
}

Actual result:
--------------
object(DateTime)#1 (3) {
  ["date"]=> string(26) "2015-09-01 00:00:00.000000"
  ["timezone_type"]=> int(1)
  ["timezone"]=> string(6) "+00:00"
}
object(DateTime)#1 (3) {
  ["date"]=> string(26) "2015-08-31 15:00:00.000000"
  ["timezone_type"]=> int(1)
  ["timezone"]=> string(6) "-09:00"
}
object(DateTime)#1 (3) {
  ["date"]=> string(26) "2015-08-31 15:00:00.000000"
  ["timezone_type"]=> int(1)
  ["timezone"]=> string(6) "+00:00"
}
object(DateTimeImmutable)#2 (3) {
  ["date"]=> string(26) "2015-09-01 00:00:00.000000"
  ["timezone_type"]=> int(1)
  ["timezone"]=> string(6) "+00:00"
}
object(DateTimeImmutable)#3 (3) {
  ["date"]=> string(26) "2015-08-31 15:00:00.000000"
  ["timezone_type"]=> int(1)
  ["timezone"]=> string(6) "-09:00"
}
object(DateTimeImmutable)#1 (3) {
  ["date"]=> string(26) "2015-08-31 15:00:00.000000"
  ["timezone_type"]=> int(1)
  ["timezone"]=> string(6) "+00:00"
}