I Found it!!!

http://en.wikipedia.org/wiki/Glicko_rating_system

Both Glicko and Glicko-2 rating systems are under public domain and found implemented on game servers online (like Free Internet Chess Server, Chess.com, **Pokémon Showdown!**, Nodewar and SchemingMind). The formulas used for the systems can be found on the Glicko website.

Where t is the amount of time (rating periods) since the last competition and '350' is assumed to be the RD of an unrated player. If several games have occurred within one rating period, the method treats them as having happened simultaneously. The rating period may be as long as several months or as short as a few minutes, according to how frequently games are arranged. The constant c is based on the uncertainty of a player's skill over a certain amount of time. It can be derived from a thorough data analysis, or estimated by considering the length of time that would have to pass before a player's rating deviation would grow to that of an unrated player. If it assumed that it would take 100 rating periods for a player's rating deviation to return to an initial uncertainty of 350, and a typical player has a rating deviation of 50 then the constant can be found by solving 350 = \sqrt{50^2 +100c^2} for c.[1]