Recently, I have started seeing some of my early puzzles as the bonus puzzle. When I first came here in 2011, the latest bonus puzzles were from 2005. Through the years, I saw problems with later and later dates. I started seeing puzzles from 2006, 2007, 2008, 2009, and 2010. Now, I have begun to see puzzles from 2011, so some of them are by me. What is the rule for how long ago a puzzle must be to be the bonus problem?