Timestamps allow us to identify a date and time when a piece of data existed or an event took place. For example, we use timestamps to establish the date when we grant a patent. Services that offer trusted timestamps on the blockchain exist, where one creates a timestamp on a value by sending the blockchain a transaction containing the value, which is eventually confirmed in a block a miner creates and timestamps. Our work focuses on the accuracy of such timestamps created on the Ethereum blockchain. We estimate that their mean accuracy is half of the average time between consecutive blocks. However, Ethereum clients geth and parity allow accuracy to vary in up to a few tens of seconds. Surprisingly, we found out empirically that miners can backdate data using the timestamp on a block, which is a serious flaw in timestamping services. To address this problem, we propose a new decentralized timestamping service that combines smart contracts and distinct time providers. The service offers timestamps with higher accuracy of milliseconds. We conduct a cost analysis of our service, which shows that Ethereum transaction fees drive our timestamps costs when the service scales up. Finally, we discuss alternatives to cut costs.

时间戳使我们能够识别数据存在或事件发生的日期和时间。例如，我们使用时间戳记来确定授予专利的日期。存在在区块链上提供可信时间戳的服务，其中一种服务是通过向区块链发送包含该值的交易来为值创建时间戳，该交易最终会在矿工创建的区块和时间戳中得到确认。我们的工作重点是在以太坊区块链上创建的此类时间戳的准确性。我们估计它们的平均准确度是连续块之间平均时间的一半。然而，复仇客户GETH和奇偶校验允许精度在几十秒内变化。出乎意料的是，我们凭经验发现矿工可以使用块上的时间戳来回溯数据，这在时间戳服务中是一个严重缺陷。为了解决此问题，我们提出了一种新的分散时间戳记服务，该服务将智能合约和不同的时间提供者结合在一起。该服务提供具有更高毫秒精度的时间戳。我们对服务进行成本分析，结果表明，以太坊交易费用会在服务扩展时推动我们的时间戳成本。最后，我们讨论了降低成本的替代方法。