"If $timezone is omitted or null, the current timezone will be used." - note, that timezone IS NOT equal offset, if its important for your application.
If default timezone = Europe/Moscow, then:
echo (new \DateTimeImmutable('2014-10'))->format(DATE_ATOM); // gives "2014-10-01T00:00:00+04:00"
echo (new \DateTimeImmutable('2014-11'))->format(DATE_ATOM); // gives "2014-11-01T00:00:00+03:00"
because of law changes (abolition of "summer time").