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Suppose that in my solidity contract, I need to see the price of ETH in terms of USD at a specific time, say on January 1, 2021. I would like to use the Historical Price Data Feed from chainlink (URL below). I imagine that to achieve this, I would have to pass the roundId as a parameter when calling the outermost function, so that the contract can verify that this roundId indeed gives the price corresponding to the timestamp of January 1, 2021. I am happy with this construction. However, I do not entirely understand the data given back. How do I verify that the roundId indeed corresponds to the timestamp of interest?

URL to the documentation: https://docs.chain.link/docs/historical-price-data/

Interface:

function latestRoundData() external view 
    returns (
        uint80 roundId, 
        int256 answer, 
        uint256 startedAt, 
        uint256 updatedAt, 
        uint80 answeredInRound
    )

Chainlink explanation with these return arguments:

  • roundId: The round ID.
  • answer: The price.
  • startedAt: Timestamp of when the round started.
  • updatedAt: Timestamp of when the round was updated.
  • answeredInRound: The round ID of the round in which the answer was computed.
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The startedAt and updatedAt values returned will give the time (unix epoch) that the round was started and submitted. You can compare this to your 'Jan 1 2021' value

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  • Ok, so how can I verify that a roundId corresponds to the given timestamp? Also, how do I find the correct roundId off-chain? Jul 8 at 11:13
  • Obviously it is very unlikely that a roundId timestamp corresponds exactly to the date of interest... Jul 8 at 11:14
  • it's hard to find the roundId that corresponds to a given timestamp unless you do a lot of iterating off chain. But unfortunately the way the data is stored on-chain, it's not easy to just plug in a date/time and get a price back. There should be a blog post coming out in the next few weeks that addresses how to solve this exact issue. Jul 29 at 2:30
  • Thanks a lot for taking the time to respond. In fact, we would be happy with an off-chain solution such as the one you describe. How would this work exactly? How can we find the correct roundId corresponding to a timestamp, and how can we verify correctness on-chain? Jul 30 at 11:45
  • I'll be working on exactly this later this week, I have a couple of rough ideas in my head, but don't want to respond unless I know they work. Will response back here once I've got a basic example working Aug 2 at 1:59

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