How do bitcoin clients agree on what time it is?

[nimda]

In order for the difficulty to be changed every 2016 blocks, the clients need to know how long it has been since the last retarget. Clocks aren't perfect, though, and there could be malicious clients. How do nodes agree?

[1713850286]

1713850286

[Revalin]

https://en.bitcoin.it/wiki/Block_timestamp

Timestamps are checked against network time and your local time.  Nodes refuse to relay if the block's time is too far off.

[finkleshnorts]

I read a paper once, posted here on these forums, about this being a potential attack vector.

[Stephen Gornick]

I read a paper once, posted here on these forums, about this being a potential attack vector.

Probably here:

 - http://bitcointalk.org/index.php?topic=43692.msg521772#msg521772

[nimda]

Does this mean that a large miner or a pool op could timestamp blocks either at the minimum or maximum variance allowed to manipulate the difficulty?

[Revalin]

Yes, a corrupt operator can slightly manipulate the timestamps for a marginal benefit.

Doing a it would in a big way would require a significant 51% attack where considerable amounts of history were rewritten in order to achieve a moderate benefit. It is theoretically a problem, but the risk/cost/reward is poor.  As such I don't think anyone has ever bothered.