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I am looking for a website that allows me to retrieve the time at which each difficulty change happened. The time doesn't need to be accurate so a time stamp from the block will do. I want to fetch this information automatically every now and then, but I can't seem to find a webpage that provides an API for this.

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http://blockexplorer.com/q/nethash/2016 provides the necessary information and shouldn't be too hard to parse. Every row corresponds to the first block at a given difficulty.

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    You can also use blockexplorer.com/q/nethash/2015 – theymos Nov 30 '11 at 21:00
  • This should probably be blockexplorer.com/q/nethash/2016 . The interval between successive retargets is 2016 blocks, and this also gives the correct offset - every row is the first block of every difficulty. I've edited the answer to use this improvement. – Meni Rosenfeld Dec 1 '11 at 6:52
  • Actually, I think that the last block of each difficulty is at 2015. This means that blockexplorer.com/q/nethash/2015 has the correct timestamp, but I have to use the difficulty of the following block. It's a little tricky... – nmat Dec 1 '11 at 9:06
  • /2015 isn't good because it's not the right interval. If you want the last block of every difficulty you need 2015, 4031, 6047... But /2015 will give you 2015, 4030, 6045... . Unless there's a parameter for nethash to control the offset (or theymos is willing to add one) it can't be used for this. But you can query each block separately with e.g. blockexplorer.com/b/155231 . Anyway, why do you want the timestamp of the last block of a difficulty rather than the first block of the next difficulty? – Meni Rosenfeld Dec 1 '11 at 13:48
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    @nmat: This makes sense, you do want the block at the -1 offset. But you still need a 2016 interval, which /nethash/2015 doesn't provide. If you don't need it very accurate you can just use /2016, and to gain a bit more accuracy you could subtract 10 minutes which is the average difference between this block and the previous one. – Meni Rosenfeld Dec 1 '11 at 18:58

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