Evert Pot has a new post sharing some of the changes in DateTime handling that he's updated in the latest release in the PHP 5.5.x series.
PHP 5.5.10 got released a few weeks ago, and among other things, it added some new functionality related to timezone handling. In short, [subtracting from UTC] now works. Normally this would not be recommended, as you really should specify timezones based on their geographical location. This information is not always available though, so it's a welcome new feature.
Other changes include the removal of the automatic translation from "UTC" to "GMT" as well as errors being thrown when one of the "odd" timezones are used (he provides the list). Additionally, an update around timezone "guessing" has been added and the fallback that was in place has been removed.