Trying to find a young adult fantasy book I read in middle school. The main character is a young boy. He either runs away or is enticed onto a property that is surrounded by a big wall. While he is there, he finds that each day all of the kids there celebrate their birthdays and holidays. He is there for roughly a month. He is not allowed to leave. When he does finally escape, he finds out that each day he spent inside those walls was actually a full year outside of those walls.