OpenHistoricalMap/Projects/United States/Time zone boundaries

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The evolution of United States standard time zone boundaries from 1919 to 2024 in five-year increments.

This project has added complete coverage of time zone boundaries to OpenHistoricalMap, with coverage of every official time zone boundary change since the United States adopted a nationwide system of definite time zone boundaries on January 1, 1919. These boundaries are not to be confused with data on the standard time observed by each region, which is much more complex and not as well suited to OpenHistoricalMap's data model.




Unlike many countries, the United States observes standard time according to a system of time zone limits that are defined in a similar manner as administrative boundaries. These boundaries usually align with state and county lines but occasionally depart from them, so that a time zone is a geographic construct requiring the use of a map. The boundaries have changed numerous times since 1919 and presently are based in part on historical features such as survey lines and abandoned railways,[1] making them an ideal subject for OpenHistoricalMap.

As a side benefit, the boundaries extend into the waters surrounding each state, up to the legal limit of each state. This will help us patch up the state boundaries imported from the Newberry Library dataset, which incorrectly hug the coastline and omit entire states and territories.


This project will mostly rely on works by the United States Department of Transportation, which are in the public domain under federal law:

For the period between March 30, 1924, and April 3, 1927, when the boundary between the Eastern and Central Time Zones incorporated the Columbus, Ohio, city limits by reference, we can use the following work as a reference:

For the period between April 3, 1927, and September 27, 1936, when the boundary between the Eastern and Central Time Zones incorporated the Cincinnati city limits by reference, we can use the following works as references:

Prior art

Editorial considerations

Most sources available on the Internet, including the tz database and OpenStreetMap, model time zones as something akin to annexation boundaries: if one time zone loses territory to another, that territory is carved out as a separate "time zone". Yet these sources generally do not account for changes in the administrative boundaries from which the time zone boundaries are derived, so an OSM-based map is insufficient to reliably determine historical standard time observance in some locations. By contrast, OpenHistoricalMap represents boundary changes as a time series of coherent boundaries overlapping each other, each representing the whole territory during a given time period. The resolved boundaries will facilitate point-in-time visualizations, while an all-time overview would require some postprocessing.

Additionally, tz and OSM conflate changes in daylight saving time (DST) observance with time zone changes. However, the area in which DST is observed does not really constitute a kind of boundary per se; it is an attribute of another administrative boundary or, in some cases, an organization such as a railroad or park operator. Therefore, this project will ignore local and state DST observance. A data consumer that wishes to determine DST observance will need to consult an external database, such as Wikidata. Conversely, the focus on observed time causes the tz database to ignore many well-documented time zone boundary changes because communities along the boundaries reportedly already observed de facto times consistent with the boundary to which they were being moved – usually the reason for moving the boundary, after all. Although OHM would strive to map a de facto boundary in addition to a de jure one in case of a dispute, these are not disputes about where a boundary runs, only at most a dispute about where it ought to run.

The tz database aims to cover changes since the Unix epoch of January 1, 1970, ignoring any earlier changes except occasionally as supplementary data. Nevertheless, there are some omissions even given this limited scope. For example, the eastern fourth of Kearny County, Kansas, corresponding to Unified School District No. 216, was moved to the Central Time Zone in March 1970; it would be another 20 years before the rest of Kearny County would follow it to the Central Time Zone. [2][3] This should in theory result in two separate "time zone" entries for "America/Lakin" and "America/Deerfield", but as of March 2024, these entries have not yet been added to the tz database. [4] OHM will include changes all the way back to 1919. To the extent that any portion of a boundary is defined vaguely in the regulations, the boundary relation will include a way tagged with indeterminate=yes indefinite=yes or a note=*.


Bulk import

The first step will be to transform the USDOT time zone dataset into OSM format:

  1. Download the entire FeatureServer using QGIS.
  2. Export the layer as a GeoJSON file.
  3. Import the GeoJSON file into OpenJUMP.
  4. Convert the layer into a planar graph.
  5. Export the edges as a GeoJSON file.
  6. Import the GeoJSON file into JOSM.
  7. Remove all the tags from the 191 ways.
  8. Clean up some artifacts originating from the dataset, such as a 280-by-1 12-foot (85.34 m × 0.46 m) hole in the Central Time Zone on the Nebraska–South Dakota line.
  9. Manually create nine boundary relations and add the ways to them.
  10. Manually create nine chronology relations and add the boundaries to them.
  11. Split ways at state and county boundaries according to the TIGER boundary layer, in order to avoid overlong ways.
  12. Conflate overland boundary ways with existing county boundary ways from the Newberry import and merge in newly created ways over water.

The dataset's attributes will be transformed as follows:

Tags on boundary ways
N/A source=
Tags on boundary relations
N/A type=boundary
N/A boundary=timezone
N/A start_date=2007-11-04
N/A source=
Zone: Eastern name=Eastern Time Zone
name:en=Eastern Time Zone
wikipedia=en:Eastern Time Zone
UTC: -05:00 utc=-05:00

This tagging scheme conforms to the tagging scheme used in OSM. However, this import will not set the timezone=* key, because tz database entries are orthogonal to the actual time zones we are mapping.

Post-import cleanup

After uploading, the next step will be to manually extend existing state and county boundaries to include the newly imported boundaries over water. This will need to be a manual step post-upload because the existing data is too messy to conflate automatically, with state boundaries that generally don't already connect to county boundaries and generally have less accurate geometries anyways. There is too high a risk of edit conflicts if we attempt to conflate existing features as part of a bulk nationwide upload. In the Atlantic, Pacific, and Gulf of Mexico, the generally 3-nautical-mile (5.6 km; 3.5 mi) boundaries will be conflated with new copies of each state boundary dating from the signing of the Submerged Lands Act on May 22, 1953.[2][3] The seaward boundaries in the DOT dataset are likely just approximations; later, we can conflate them with the dataset.[4]

Further mapping

After conflation is done, the final step will be to work backwards from the present to 1919 based on the Federal Register notices and ICC reports listed in the sources, creating additional copies of each boundary as necessary.


Please add your wiki and/or OHM user name to this list if you are interested in contributing to the project:


  • OOjs UI icon check-constructive.svg Import USDOT dataset of current time zone boundaries
  • OOjs UI icon check-constructive.svg Conflate with county boundaries over land
  • Conflate with county boundaries over water
  • Conflate with state boundaries over water
  • OOjs UI icon check-constructive.svg Work backwards to 2000
  • OOjs UI icon check-constructive.svg Work backwards to 1970
  • OOjs UI icon check-constructive.svg Work backwards to 1967
  • OOjs UI icon check-constructive.svg Work backwards to 1919
  • Adjust boundaries according to municipal boundaries as specified in 49 CFR 71.5(h), 49 CFR 71.7(g), and 49 CFR 71.9(d)
  • Adjust boundaries according to railroad exceptions under ICC administration
  • Import the rest of the Columbus city limits beyond the parts that formed the Central–Eastern boundary [5]

Notes and references

External links