How does that happen? Obviously the miner of 826 built his block on top of 825, but why is the time 1 second earlier?


There is no requirement that blocks have a timestamp after the previous block. The only requirement is that the timestamp is greater than the median timestamp of the last 11 blocks. So this means that a block can have a lower timestamp than its parent, within a certain bound.

This happens because miners do not have perfectly in sync clocks. There can be slight differences in their internal clocks that make them off by a few seconds. If a miner gets really lucky and finds a block soon after another one, because of the clock differences, it is possible that his clock is still behind the timestamp of the parent block. That is probably what happened here.

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There is a fair amount of leeway in the block timestamp. The timestamp for block N must be greater than the median network time, which is calculated as the median of the past 11 blocks, and also less than the network time + 2 hours, where network time is calculated based on the node's system time, as well as the median time reported by the node's peers.

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