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Imagine there are a series of reorgs happening because of maybe some fee sniping event or something else, doesn't matter.

If that happens for over than 2 hours, then new blocks will not be able to mined with their current timestamp anymore, otherwise they wouldn't be accepted? In such a case will the chain have to proceed with invalid block times forever after the reorgs end?

Example:

block 100: 00h00
block 101: 00h10
block 102: 00h20
block 103: 00h30
  :: reorgs happen for 3 hours, many blocks discarded ::
block 104: 03h30 # invalid because of the timestamp.

I think the question is if whoever is mining the block 104 at the end will have to fake a timestamp in the past (like 01h30) even though its actual current time is 03h30.

Murch
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fiatjaf
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  • By the way, if this issue existed, we would have encountered it on block 1: https://bitcoin.stackexchange.com/a/77964/5406 ;) – Murch Sep 28 '23 at 15:33

3 Answers3

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No.

There are two restrictions on the timestamp of blocks:

  • A consensus rule that the timestamp of a block must be strictly larger than the median timestamp of its 11 predecessors (parent, grandparent, great-grandparent, ...). This is a property of just the block itself - it's either valid or not, independent of when validation happens.
  • An acceptance rule that the timestamp cannot be more than 2 hours in future (compared to the verifier's local clock). Since this depends on the local clock, this property can change over time (and once enough time passes, a block formerly found to be invalid by this rule may become acceptable).

Neither of these rules tightens when a reorg happens. The first rule widens (as the reorged out blocks disappear from the median timestamp calculation), and the second rule is unaffected (it continues to allow more and more as time passes).

Pieter Wuille
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  • Either my question was unclear or I don't understand this answer.

    EDIT: I now understand the answer.

     – fiatjaf Sep 16 '23 at 12:26
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No. The 2-hours rule is checking if the timestamp of a block is not more than 2 hours in the future. A valid timestamp for a given tip cannot become invalid as time passes.

I think you are confusing the 2-hours rule with the MTP rule.

From the example you added in your edit, block 104 is still valid according to the MTP rule since it's based on the timestamps of the previous blocks. And it is still valid according to the 2-hours rule since it's compared to the node's clock at the time of reception.

Antoine Poinsot
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  • My question is about blocks being mined now -- but such that they would be invalid according to the chain. I'll improve the question. – fiatjaf Sep 16 '23 at 12:20
  • @fiatjaf I think you're still confusing the two rules, see my answer. The 2-hour rule is w.r.t. the verifier's local clock, not w.r.t. to other blocks' timestamps. – Pieter Wuille Sep 16 '23 at 12:26
  • Reading your edit, i still think you are confusing the MTP and 2-hours rule. I'll edit my answer to use your example. – Antoine Poinsot Sep 16 '23 at 12:26
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    Yes, that was it. I didn't realize the 2-hours rule was according to the current time of the validator. – fiatjaf Sep 16 '23 at 12:27
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My understanding of the 2-hour rule was incorrect. I thought the 2 hours was in relation to the timestamp of the previous block, but it is in relation to the timestamp of the local clock of whoever is verying the block at any time, so miners don't have to fake timestamps at all, they can just use their current local time and it should be correct if their clocks are correct.

fiatjaf
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