How to sync clocks to Bitcoin's timestamp?

[mczarnek]

I had an interesting thought, given that every block in Bitcoin has a timestamp attached to it, is there a way to use the timestamps plus the time that you saw an average block in order to sync your clock to the time on the network?  What if someone fakes the timestamp and fudges a little bit, can you at least stay within say 15 seconds assuming the majority is honest and it takes 10 seconds for a block to propagate to 75% of the network?

On a similar note, how do Proof of Stake blockchains agree to time?  After all many of those algorithms involve a time component based on the time since the network saw the last block.. in those cases does the network need to have it's clocks in sync in order to mine together? Or can they work together to agree on a time based on blockchain info?

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[shorena]

I had an interesting thought, given that every block in Bitcoin has a timestamp attached to it, is there a way to use the timestamps plus the time that you saw an average block in order to sync your clock to the time on the network?  What if someone fakes the timestamp and fudges a little bit, can you at least stay within say 15 seconds assuming the majority is honest and it takes 10 seconds for a block to propagate to 75% of the network?
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Timestamps or modified as part of finding a valid block, they are not reliable and do not have to be incremental either. Block n can have a later timestamp than block n+1 as long as its within certain bounds.

[fbueller]

A blocks median-time-past IS guaranteed to increase (there is a proposal to have locktime use this instead of that included in a block header), so it may still work.

[shorena]

A blocks median-time-past IS guaranteed to increase (there is a proposal to have locktime use this instead of that included in a block header), so it may still work.

If you are fine with a clock that is correct within a 2 hour limit, sure.