Time zones change over time

[infinity0]

Time zones change over time. And I don't mean simply daylight savings, but the actual polygons defining what zones are in effect at a given (lon,lat) location on Earth.

This is why the Google Time Zone API requires you to pass in a date+time when you query it - https://developers.google.com/maps/documentation/timezone/overview

This is relevant if I want to find out the local time zone of some old photos that I have UTC timestamps and GPS co-ordinates for - I don't want to use the current IANA time zone definitions, but the old definitions that were in effect when I took that photo.

Unfortunately this library does not provide me with this sort of API.

Other libraries are also affected, e.g. jannikmi/timezonefinder#378

[ringsaturn]

Hi there,

I’m aware of this. The Python binding project only provides the most recent data, not historical data.

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If you need historical data, you should look at https://github.com/evansiroky/timezone-boundary-builder, which contains historical boundary datasets.

Originally in the tzf project, the Go version (https://github.com/ringsaturn/tzf) has the ability to initialize a finder based on another dataset:
https://pkg.go.dev/github.com/ringsaturn/tzf#NewFinderFromPB.

This requires significant work to process and distribute the large dataset.
Each tzdb release produces about 10 MB of reduced data, and the full dataset is around 90 MB.

I’ll leave this issue open to see whether others have similar use cases.

[ringsaturn]

By the way, a friendly reminder: the Google Maps Time Zone API works differently from what you described. The timestamp parameter is used to determine the time zone offset at that moment (including whether daylight saving time is in effect).

Note that the API does not take historical time zones into account. That is, if you specify a past timestamp, the API does not take into account the possibility that the location was previously in a different time zone.

Source: Time Zone requests and responses

[infinity0]

OK thanks for looking into that, that's a shame as some other online commentators (incorrectly apparently) assert that it is how the API works.

But I dug into it a little bit further however and according to AI and also its own website, the IANA TZ DB is supposed to define historical time zone boundaries at least after 1970.

https://www.iana.org/time-zones:

The Time Zone Database (often called tz or zoneinfo) contains code and data that represent the history of local time [emphasis added] for many representative locations around the globe. It is updated periodically to reflect changes made by political bodies to time zone boundaries [emphasis added], UTC offsets, and daylight-saving rules. Its management procedure is documented in BCP 175: Procedures for Maintaining the Time Zone Database.

https://www.rfc-editor.org/rfc/rfc6557.html:

To be clear, the TZ Coordinator SHALL NOT set time zone policy for a
region but use judgment and whatever available sources exist to
assess what the average person on street would think the time
actually is, or in case of historical corrections, was.

I am unfamiliar with the technical details of how the DB is structured so have not yet spent the time to figure out how to extract this information from the DB. In particular the zdump tool only outputs time zone DST changes, not its actual geographical GPS polygon changes.

[infinity0]

More info here:

• https://stackoverflow.com/questions/65776848/get-historical-timezone-info-for-a-location (incorrectly asserts Google API has historical info)
• https://stackoverflow.com/questions/16086962/how-to-get-a-time-zone-from-a-location-using-latitude-and-longitude-coordinates/16086964#16086964 more online APIs

This looks promising:

https://learn.microsoft.com/en-us/rest/api/maps/timezone/get-timezone-by-coordinates?view=rest-maps-2025-01-01&tabs=HTTP

Use to get the current, historical, and future time zone information for the specified latitude-longitude pair.

The Get Timezone By Coordinates API is an HTTP GET request that returns current, historical, and future time zone information for a specified latitude-longitude pair. In addition, the API provides sunset and sunrise times for a given location, with the exception of regions that observe solar days.

[ringsaturn]

References on history timezones:

• Migrate heuristics to Wikidata evansiroky/timezone-boundary-builder#201

[evansiroky]

Regarding the claim that the polygons change over time, can a verifiable use case be provided that shows where this is happening? The data from timezone-boundary-builder does come with different OpenStreetMap updates, but I feel that most of the time, the polygon changes add to an existing set of on-the-ground borders that should have been in place, but were not until the edits were made. I could see a possible use case of one area using a particular timekeeping method and then switching to another known method, but most of the time changes like these result in entirely new timezone IDs that created such as Asia/Famagusta. Therefore, it would be helpful if a verifiable use case could be provided that substantiates the need for the added complexity of having different polygons for different points in time.

[clawdney]

Hi! I'm interested in helping with this. Could you share more context about the use case?

[infinity0]

Regarding the claim that the polygons change over time, can a verifiable use case be provided that shows where this is happening?

https://en.wikipedia.org/wiki/List_of_national_border_changes_(1914%E2%80%93present)