When local mean time ended

By Blake Patterson · Published October 27, 2026 · Data: IANA tz database 2026f

The tz database starts 298 of its 312 zones with a period of local mean time: the clock set by the average position of the sun at the reference city, given to the second. Half of these zones had left it by 1901, and the last ones held on until 1952.

What local mean time is

Before standard time zones, a town kept its own time, based on the sun. Noon was when the sun stood highest on average over the year, so each place’s clock differed from its neighbours’ by four minutes for every degree of longitude. The database records this as an offset from UTC with seconds, such as UTC−0:01:15 for London or UTC−4:56:02 for New York, worked out from the longitude of the reference city. It is a reconstruction for the record, not a figure taken from a historical document.

Everything in this article comes from the part of the database before 1970, which its maintainers describe as a best effort. The dates are the database’s, and for many zones they are the date of the first known official standard time rather than an exact day on which every clock in the area changed.

The great switch of the 1880s and 1890s

The busiest decade was the 1890s, with 62 zones, and the two decades from 1880 to 1899 together account for 120 zones, about 40 percent of the total. The busiest day with a precise date is November 18, 1883, when 19 zones in the United States left local mean time together. Many other records give January 1 as the date: 23 zones on January 1, 1890 and 22 on January 1, 1880. A second wave, from 1910 to 1929, covers 108 zones, led by Asia (41), North America (22) and South America (19).

The first and the last

United Kingdom is the earliest in the record to leave local mean time, in 1847. The other early cases are in Czech Republic, Switzerland, India, Italy, Indonesia and New Zealand.

At the other end, a few zones kept local mean time well into the twentieth century. Niue and Cook Islands are the last, in 1952; Saudi Arabia and Bhutan follow in 1947.

Moves across the date line

A few records change their local mean time before they ever adopt a standard offset. These are moves across the international date line, in which the clock time stays the same but the date shifts by a day. 16 zones show one. In 1844 Manila, Guam, Palau and Kosrae moved from the American side of the date line to the Asian side; in 1867 7 Alaskan zones moved the other way, so their local mean time went from almost 15 hours ahead of Greenwich to about 9 hours behind.

How far the old clocks were from today’s

Comparing each zone’s last local mean time with its standard time today shows how much the switch moved the clock, ignoring later summer time. The median difference is 27 minutes. In 58 zones it is more than an hour, and the largest gaps, after setting aside whole days gained or lost across the date line, are in Dawson (2 hours 18 minutes), Nome (2 hours 2 minutes), Whitehorse (2 hours) and Coyhaique (1 hour 48 minutes). Where the difference is large, today’s clock is set from a meridian far from the city: these are the same places that appear among the largest gaps in the article on clocks and the sun.

At the other end, 38 zones adopted a standard time within five minutes of their local mean time, so the switch barely moved their clocks. London is one of them: its local mean time was only 1 minute behind Greenwich.

Zones without local mean time

14 zones have no local mean time in the record. 14 of them begin with a period marked as having no local time at all: research stations in Antarctica and settlements in the Canadian Arctic, which the database treats as uninhabited until the date they were set up. Their records start directly with a standard offset.

Each zone page gives its own local mean time and the date it ended, where the record has one, and the article on clocks and the sun shows how far today’s clocks have moved from it.

Source: IANA Time Zone Database, release 2026f (public domain), computed by this site; see about the data for definitions. Look up any zone in the converter.