Why is Chrome 67 Returning Inaccurate Time Zone Offsets for Dates Before 1900?

Historic Time Zone Changes in Chrome 67: A Call to Action

Introduction:
Recent updates to Chrome 67 have raised concerns among developers regarding inaccuracies in time zone calculations. This article explores the issue, its potential implications, and provides guidance for mitigating its impact.

The Problem:
After updating to Chrome 67, users have encountered discrepancies in the output of the getTimezoneOffset() method when applied to dates prior to 1900. Specifically, Chrome 67 returns a different time zone offset compared to other browsers for dates in that period.

Analysis:
The culprit behind this discrepancy lies in the underlying time zone data used by Chrome. In previous versions, Chrome relied on the operating system's time zone information. However, with Chrome 67, it appears that the browser has begun bundling its own IANA time zone data, which differs from the data provided by operating systems.

The IANA time zone database contains historical timezone data, including those that existed prior to 1900. It is important to note that this data is not guaranteed to be accurate for all times and locations, as it is challenging to gather comprehensive historical information.

Consequences:
This discrepancy can significantly impact applications that rely on accurate time zone information for historical data. Code that assumes a consistent time zone offset for all past dates may now produce incorrect results. The implications are particularly worrisome for applications handling dates close to the 1900 boundary.

Mitigation:
To mitigate the impact of this issue, developers are advised to take the following steps:

• Avoid querying time zone information prior to 1970. The IANA database acknowledges that its pre-1970 data may be unreliable, so it is best to avoid relying on it.
• Update code to handle time zone changes. Applications that need to deal with historical dates within the pre-1900 period should be updated to take into account potential time zone adjustments.
• Consider using a time zone library. Third-party libraries can provide more accurate and comprehensive time zone information, including historical data.

Conclusion:
The recent changes to Chrome's time zone handling have highlighted the importance of considering the accuracy and reliability of historical time zone data. By implementing the recommended mitigation steps, developers can ensure the continued reliability of their applications as they navigate the complexities of historical time zone transitions.

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