Time Zone History

About the data

Every number on this site is computed from release 2026f of the IANA Time Zone Database. This page explains how a record is built, what each figure means and where the data stops being reliable.

Source and processing

The release was downloaded from data.iana.org/time-zones on 2026-10-28. The zones, their reference coordinates and the countries they cover come from the release’s zone1970.tab, zone.tab and iso3166.tab tables; alternative names come from its Link lines. Clock transitions were read with Python’s zoneinfo module from the compiled files of the PyPI tzdata package version 2026.6, which carries IANA release 2026f; the build checks that the two release names match. The base (standard) offset of every period is taken from the zone’s own lines in the source files, so daylight saving amounts are exact, including the two-hour summers of the 1940s.

Zones, records and countries

A zone is a region whose clocks have agreed since 1970. The database names each one after a reference city, for example America/Chicago. There are 312 zones in its main table. When two places have kept the same clocks since 1970, the database stores them as one zone and the second place as another name for it; the Netherlands and Luxembourg, for instance, are recorded under Belgium’s zone. Such countries are listed on the country index with the record they follow, and they have no page of their own, because their page would repeat the other one.

A country with one zone of its own shows that zone’s full record on the country page. A country with several zones of its own has an overview page and a page per zone.

Before 1970

The database aims to be correct from 1970 onward. Its entries for earlier years are a best effort: they are often based on a single source, and for places that share a zone today they show only the history of the zone’s reference city. Pages show this earlier history because it is the record the database keeps, but figures that compare zones (the counts and ranks “since 1970”) use 1970 to 2026 only.

What the figures mean

  • Standard time is the zone’s base offset from UTC, as defined in the database. A change of standard time is a change of that base offset, for example from UTC+4 to UTC+3.
  • Seasonal time (daylight saving time, summer time) is a period in which the clock is moved away from standard time for part of the year. In a few records the database defines the period below standard time as the seasonal one, and the pages follow it: Morocco (2019 to 2026), Western Sahara (2019 to 2026), Namibia (1994 to 2017), Ireland (1971 to 2037), Czech Republic (1946 to 1947).
  • Clock changes count every moment the local clock jumped, forward or back, seasonal or not.
  • Local mean time is the time kept by a place before it adopted a standard offset; the database gives it to the second, from the longitude of the reference city.
  • Mean solar time gap: local mean solar time at the zone’s reference point is its longitude times four minutes. The gap is today’s standard time minus that value. A positive gap means clocks run ahead of the sun, so noon on the clock comes before the sun is highest.
  • Nearest reference points are measured along the earth’s surface between the coordinates in zone1970.tab. They compare cities, not borders.
  • Rule periods group consecutive years whose clock changes fall on the same kind of day, such as “last Sunday in March”. Times are the local clock times just before the change. Years in which the dates follow no single pattern are listed year by year.

Rankings and their scope

Every comparison names its scope. “All 312 zones” means the whole main table. A region (for example “the zones in Europe”) groups zones by the area part of their name, with the Americas split into North and Central America and South America by the country the zone belongs to. “The zones in Russia” means the zones the database assigns to that country. “The zones on UTC+1 standard time” groups zones by today’s standard time. Ties are reported as ties.

Dates after 2026

Clock changes for 2027 and 2028, and converter results up to 2037, follow the rules in release 2026f. They are calculations, not announcements, and change whenever a government changes its rules. The site will be rebuilt from later releases.

How to read a record page

The first lines name the zone’s most distinctive facts, then the rule in force. The large chart has one row per year since 1970 and one column per day; its colours compare the clock on that day with today’s standard time, so a zone that never changed is one flat colour. A narrow strip above it shows standard time before 1970. Below come the table of every change of standard time, the daylight saving periods and the dates for 2026 to 2028.