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Can Bitcoin/blockchain be thought of as a decentralized time-stamping authority (TSA)? Is there true consensus in the Bitcoin network on a mined block's time, or just that the block was mined?

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Yes, Bitcoin can act as a time-stamping authority. However, depending on your application the accuracy of the Bitcoin blocks may not be sufficient.

Nodes only reject blocks that are greater than 2 hours into the future, and blocks created earlier than the median time of the past 11 blocks. Therefore, timestamps may be off by several hours.

also see: How accurate is Bitcoin network time?

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I don't think decentralized time-stamping authority does Bitcoin full justice, but yes blocks are timestamped and yes there is consensus on actual time. Albeit within a few hours margin. That even means that sometimes the timestamp on a newer block is further in the past than the timestamp of an older block.

  • "Albeit within a few hours margin." So, using the blockchain for priority disputes (putting a hash in a block with, e.g., Proof of Existence) could still occur if they get into the same block. "sometimes the timestamp on a newer block is further in the past than the timestamp of an older block." When has that happened? – Geremia Dec 30 '15 at 15:55
  • In the same block yes. I guess that's a reason why the blockchain makes more sense as a money application: a Bitcoin can only be spent once whether it's in the same block or not. Such a Proof of Existence hash is not limited like that. Time warps happen all the time: bitcoin.stackexchange.com/questions/3743/… – Jannes Dec 30 '15 at 16:52
  • @Geremia - it happens a ton! I was just looking at the last several weeks of block header data, there are a couple hundred blocks where timestamps are out of order - but only by a couple minutes, usually. – David Manheim Dec 30 '15 at 23:11
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    @DavidManheim If that's true, then what's the point of timestamping blocks (besides it acting as another sort of nonce)? – Geremia Dec 31 '15 at 1:12
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    @geremia just friends on how accurate your timestamps need to be. Watches are often not accurate to the second either, does that make them completely useless? Besides often you're only interested in the ordering, not the exact timestamp. Bitcoin itself only depends on the timestamps when doing the 2-weekly difficulty readjustment: it needs to correlate actual time with the number of blocks. At that scale the timestamps are accurate enough. – Jannes Dec 31 '15 at 5:33

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