We've decided to to push a breaking change to our production server, which is something we very rarely do. The reason was that some Java style thinking leaked into one of the components, the DatePicker.
When I wrote that component I was obviously thinking about something else, and I broke one of the most distinctive abstractions that we use in App Inventor: all our sequences are 1-based and not 0-based. Newcomers to programming find this a lot easier to grasp. Because Java returns month numbers on a 0-based array, I went with that and our Month attribute returned 0 for January and 11 for December.
Because the component has not been out for long, we have decided that it's better to change it now and take the heat than leaving this leaked abstraction hurting our users.
More info in this announcement in the forums, and I've added an annotation to the video and post. This break will only affect you if you are still working on an app that uses this component, or if you want to upgrade an app that you programmed in the last week, and you were using some type of correction to counteract the fact that the sequence was 0-based. Otherwise, it should not affect you (we are hoping this affects to only a few developers).
Apologies for this breaking change; it is something that should never happen (again!).