File:Evolution of United States standard time zone boundaries.webm

English: A time series map of United States standard time zone boundaries from January 1, 1919, when the federal system of time zones took effect, to January 1, 2024, animated in five-year increments. Each frame is as of noon UTC, to incorporate any changes made in the early morning hours in each time zone. However, most changes take place after January 1 in a given year, so they only appear in the following frame, as of five years later.

The time zone boundaries are shown as described in federal regulations issued by the Interstate Commerce Commission until 1970 and by the U.S. Department of Transportation since then. Many individual time zone changes are not shown because they took place sometime in between one of the five-year increments or affected too small a territory to be legible at the depicted scale.

The time zones are color-coded by UTC offsets and labeled with the abbreviations specified in 48 FR 55289. The map does not indicate observance of war time (prevailing time) or daylight saving time (advanced time), exemptions granted to railways for internal operations, or unofficial local observance of times contrary to 49 CFR Part 71 (including by units of the federal government).

Meridians 15 degrees apart indicate the theoretical ideal time zone boundaries consistent with nautical standard time zones, in order to show the gradual westward progression away from these meridians. Contemporary county, state, and territorial boundaries show how the boundaries were gradually realigned according to these boundaries instead of railways or physical features.

The main map is at a scale of 1:18,007,042 and uses an Albers equal-area conic projection (ESRI:102039). There are five insets, from right to left: Puerto Rico and the U.S. Virgin Islands (1:17,487,196, Lambert/EPSG:6566), American Samoa (1:14,489,120, Lambert/ESRI:65062), Hawaii (1:19,826,675, Albers/ESRI:102007), Alaska (1:34,371,940, Albers/EPSG:6393), and Guam and the Northern Mariana Islands (1:16,225,502, polyconic/ESRI:65161). Each inset is hidden until there is a time zone to show within it.

The time zone data depicted in this map is available on OpenHistoricalMap in the public domain under a Creative Commons CC0 dedication. See the U.S. time zone boundary subproject page for detailed sources and methodology and links to more visualizations. County boundaries are courtesy of the Atlas of Historical County Boundaries, a project of the William M. Scholl Center for American History and Culture at the Newberry Library, and also available on OpenHistoricalMap under CC0. The map data was postprocessed and styled in QGIS 3.36.1 and converted to WebM format by ImageMagick and FFmpeg.