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I mean if you can vote for a post even after the pay period (wether it be 1 or 7 or x days), it must be because you still give something somehow to the poster. Or else it would have been disabled (upvote button grayed or something like that), isnt it ?

It may still influence reputation, but there will be no more rewards. It would be great to get rewards for old posts, but I suspect it's too much work to process it