Bitcoin Forum

I understand that the bitcoin algorithm adjust difficulty by measuring the time it took to find the last 2016 blocks. What clock is used?

Here you go:https://en.bitcoin.it/wiki/Difficulty

Each miner determines his own clock.  

There are 2 "clocks" actually.

- the miner's system clock
- the median of the timestamp when peers connect

At least one of the first five peers you connect to must have a clock within 5 minutes of your computer or it displays a warning to fix your system clock.

If the median of the other peers is within 70 minutes, it uses that time as your time.

This (https://github.com/bitcoin/bitcoin/blob/master/src/util.cpp#L1259) is the code.

Each miner decides what timestamp to add to its block.  It can set it to anything but if it is more than 2 hours in the future, the block will not be accepted.

If it is to far in the past, it will also not be accepted.  A block's timestamp must be later than then median time of the 11 blocks before the block.

This gives a short time window of acceptable times. 

Thanks TierNolan!