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It's not that the rewards are reduced by time, but rather how many other people have voted for it. The first upvote gets significantly more rewards than the 100th upvote. However, in order to avoid people just upvoting everything the minute it comes out the devs introduced a "reverse auction" structure for the first 30 minutes after a post is live.

What this means is that voting on something right when it is posted actually forfeits roughly 99% of the rewards your vote would earn to the poster. After 15 minutes you only forfeit 50% of your reward. After 30 minutes, you keep 100% of the reward for yourself. The forfeited percent updates constantly as time goes on, so you forfeit varying percents for every minute in those initial 30.

I haven't read anything officially stating that, for example it's not in the White Paper, but a lot of people have been saying that. It might also just be that after 30 minutes if something has not been upvoted, it probably isn't going anywhere. But I have voted on some things that are like 30-45 minutes old that had like $50 or $100 and have gotten ok rewards for it later.

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