This rule specifies that the first block of a new period must have a timestamp no more than 600 seconds earlier than the last block of the preceding period. The periods in question are defined as lasting for 2016 blocks, which approximately corresponds to two weeks, given an average block time of 600 seconds. By setting these parameters, the proposal aims to ensure that the difference between the timestamps of the first and last blocks in any period equals exactly two weeks, thereby preventing any adjustment to the mining difficulty level within that period.

The significance of the 600-second grace period is highlighted as a countermeasure to potential exploitation through timewarp attacks. These attacks manipulate block timestamps to adjust the mining difficulty unfairly, allowing attackers to mine blocks faster than intended. The grace period effectively neutralizes one form of this attack by ensuring that even if all timestamps within a period are the same except for the last one, the overall period length remains consistent at two weeks. This is crucial because the difficulty adjustment algorithm typically examines the time it takes to mine the previous 2015 blocks, not including the 2016th block, potentially leading to a slight extension of the actual period duration beyond the intended two weeks.

However, despite the grace period's effectiveness, an additional complication arises due to the asymmetry of the Erlang distribution, which can extend the period duration by another ten minutes under a constant hash rate. This effect was acknowledged but not compensated for in the proposed rule, as detailed in a footnote within the provided GitHub link. The document suggests that while the initial effect of period extension due to the off-by-one error in block counting can be mitigated, the second effect stemming from distribution asymmetry remains unaddressed by the rule.

In summary, the proposed rule introduces a structured approach to limit the impact of timewarp attacks on blockchain networks by establishing a fixed grace period between blocks across periods. This approach maintains the intended two-week timeframe for each period, ensuring that difficulty adjustments occur as designed. Nevertheless, the proposal acknowledges but does not solve all potential vectors for exploiting timing mechanisms within the blockchain, indicating areas for further research and development to enhance network security and integrity.