Sorry for not being clear. I was talking about the iOS app. The bug appeared since the last update.
I opened the app, clicked"Add Task" at the top right corner, entered the task and then pressed "Edit Details". From there I chose "Due date > Custom". I scrolled to December 9 2012 but the text above it showed "December 10, 2012 (Sun)".
I added a new task and selected December 9, 2012 as the deadline. However, the text field mistakenly displayed "December 10, 2012 (Sun)" instead of "December 9, 2012 (Sun)".