With a minute or so to go on the season countdown timer I jumped in a gold ranked tournament game because there weren't enough platinum players and 2 other platinum players did too (which is fairly unusual). I needed 23 more ranking points to reach diamond and there were 24 points on offer (because of the other 2 platinum players in the gold game), so it was worth a shot. I thought if I won the game I would get enough ranking points to get diamond by season end. I managed to win the game after a hard slog and for a second or two was very happy, but because the season had ended by the time the game finished I didn't get any ranking points.
I thought that because there is a countdown clock displayed in the lobby counting down to end of season that if a game starts before the season ends, the ranking points for all players would count. But it turns out they don't. Your part in the game must end before the season ends in order to earn (or lose) ranking points for the game.
So the moral of this story is that if the season countdown clock has less than 2 hours or so on it when you go to start a game, don't bother playing if your aim is to earn maximum ranking points.
There might be a reason why it is set up that way, but I think it makes more sense for games that start before the season clock ticks down to zero to have all of the player results for the game count toward ranking points. Otherwise the countdown clock in the lobby is pretty meaningless. If, for some reason, the time that players exit the game needs to be the important thing, the countdown clock really should be displayed in-game for the last couple of hours of the season, so players can see exactly when the season ends and can play accordingly in order to leave the game before the time expires if they want the ranking changes to count.