Calculation of ratings

I apologise if this has been answered before. I have seen that during the course of a game, the rating of ones opponent can change quite a bit as he/she completes other games. I would just like to know which opponent rating score is used in calculating the rating change after the game. Is it the one at the start of the game when the challenge is issued or the one at the end of the game.

Originally posted by Alistair72
I apologise if this has been answered before. I have seen that during the course of a game, the rating of ones opponent can change quite a bit as he/she completes other games. I would just like to know which opponent rating score is used in calculating the rating change after the game. Is it the one at the start of the game when the challenge is issued or the one at the end of the game.

Rating calculations are based upon ratings at the time when the game ends.