When player 1 accepts to play against player 2, each player has a specific rating. Some games take many weeks to complete and both players' ratings may fluctuate. My question is the following: shouldn't the rating calculation at the end of the game be calculated using the ratings at the beginning of the game? Both players agreed the play each other based on this rating.

For example, if I agree to play a 1700 rated player and this person's rating drops to 1500 by the time we get to our endgame and I lose to him/her, then my rating will drop significantly more than if I lose to the same player who was at 1700 originally.

Swings and roundabouts, equally possible that they're rating will go up.

I think it has been mentioned before in site ideas but nothing ever came if it. I suspect it's it's much easier to program that the rating at the end of the game is counted.

However, let's say a severely underrated player starts a bunch of games at 1200. They could then start and win a slew of games and experience massive rating inflation because they're getting way more bonus points than they should.

@Jakal

Ratings are adjusted continuously as concurrent results come in. This is fair, as it more accurately reflects a player’s real / current strength than a player’s rating at the beginning of any single game. If in doubt about a player’s real strength based on his current rating, explore his previous games.

@a-unique-nickname said
Swings and roundabouts, equally possible that they're rating will go up.

I think it has been mentioned before in site ideas but nothing ever came if it. I suspect it's it's much easier to program that the rating at the end of the game is counted.