Originally Posted by dizzlmcwizzl
None of us know for sure because the formula is a secret. But my understanding is below.
All matches will count forever. However, the further you get away from any particular match they become increasingly less important. The dynamic rating is kind of like a running average that weights the more recent matches more heavily than the earliest matches.
Yes, all matches count "forever" as described, but in this specific case where a player still enters 2013 as an S due to not playing enough in 2012, I'm not sure that the 2012 matches would count. It is possible that the player enters 2013 with their "preliminary" dynamic rating from the 1 or 2 matches played in 2012 as their starting rating, or they may start with a blank slate as a 2013 S rated player. If anyone knows for sure, I'd love to know.
And regarding the count "forever", due to the way the averaging is done, once a given match is 5 or more matches old, it starts to have a very limited effect on the current rating. For example, an individual match result that is a full level (0.5) below expected by itself will only cause the rating 5 matches later to be about 0.03 lower than it would be otherwise. However, the most recent match being a full level below expected would result in a rating being 0.125 lower.