Historical timezone data for location

[wclr]

@evansiroky

Hi, there is a question probably not directly related to this package functionality.

1. If there is a problem to define a correct timezone/offset for a certain location for a particular date in the past, how this task could be solved from your prespective? Is there an available/ready to go solution for it?

2. The second problem related to the first one is to get dates of changes of the timezone/offset of a particular location (or city).

Would be grateful for your thoughts.

[evansiroky]

I believe this use case of "what is the historical timezone identifier of a particular GPS coordinate" doesn't have a good example of why it is needed to be answered yet that I am aware of. For use cases where the time differed in the past at a certain GPS coordinate, the timezone database's rules should provide satisfactory logic for determining what the time was given a timezone identifier which when combined with the data from timezone-boundar-builder, which this package uses, can determine what the time was in a certain geographic area.

There have been changes to the timezone-boundar-builder data and position of borders which I believe have fallen in a few categories:

1. Correction/improvements to administrative boundaries that have shifted boundaries slightly
2. Improvements to the overall composition of timezone relations based on what OSM contributors have determined
3. Completely new zones that were added to the timezone database (see Asia/Famagusta for example) which created a new zone which resulted in the loss of area of a previous zone. In all of these cases, the old time history up to the point of the new zone being created are noted within the timezone database.

Note that none of these changes includes shifting timezone boundaries due to a region of people beginning to adopt a new timekeeping method. Such a change might turn out hard to track if only a few people begin observing the new time, especially if the change isn't made official by a decree of a government entity. However, given the presence of zones like America/North_Dakota/Center, I sort of expect that new zones in the timezone database would get created to cover use cases of even a small region of people changing the timekeeping method they observe.

I've had this question come up before, so please let me know if this interpretation of the desired use case is not properly capturing what you're thinking. In short, I believe the existing timezone database identifiers and the boundary definitions should already provide enough information for the use case of "what time was it at this GPS coordinate in the past?"

[hwinkler]

Evan, thank you for this great work.

In minerals development you find historical records, like drilling logs, that record events in their local time. You might have a mix of electronic and paper records. If the paper record says an overpressure event happened at 6 am on Nov 24, 2009, and you have an electronic recording of measurements with GMT timestamps, you want to display those GMT times for easy comparison to those paper records. To do that you need to know the TZ offset in effect for that geographic location, on that day.

Apologies if I missed something in your explanation -- if your explanation covers how one could do that, I need to understand better.

[evansiroky]

@hwinkler, I believe this use case seems possible given the available tools. In this case, you want to know the history of timekeeping occurring at GPS coordinate. The timezone database gets updated so that changes in timekeeping methods are tracked within new zones. Therefore, the timezone identifier you receive from this library for a GPS coordinate should already be able be used to derive the TZ offset for a particular date using a 3rd party library that can do timezone-aware datetime conversions.

I recommend becoming familiar with how the timezone database project works. This project is continually updating its data that describes timekeeping and time changes. For example of how it worked in action, I recommend reading this entry in the NEWS about how a new timezone for America/Ciudad_Juarez was created in the year 2022.