TimeZones WorldWide Sunday 20th July 2025 using AEST

Via sun dials and pendulum clocks, the phrase "Mean solar time" was introduced in nautical almanacs in England in 1834 and in France in 1835.

It averaged the 24*60*60=86400 seconds mean solar day over a Gregorian calendar year of 365.2425 days, recognizing that a sun dial's apparent time ran behind mean time by about 14 minutes near February 6, then up, then down, then up by about 16 minutes near November 3. Its mean second became a fixed unit of time....

An average solar day is 24 hrs. This is the time that we set our clocks to. At perihelion (4th January 2025) the Earth is travelling at its fastest around the sun. Because the sun is moving faster relative to the background stars, it will take longer for the Earth to "catch up" reach solar noon anywhere, resulting in a fractionally longer day. At aphelion (4th July 2025) the Earth moves at its slowest pace around the sun, so the day is fractionally shorter.

10 means 24 hours 10 seconds, 20 means 24 hours 20 seconds, -10 means 23 hours 59 minutes 50 seconds, etc.
The blue line gives the difference due to the tilt of the Earth’s axis.
The red line gives the difference due to the ovalness of the Earth’s orbit.
The black line gives the overall effect, combining the blue and red lines.

Apparent solar days are also fractionally longer than mean days in June and December (solstices) over the two tropics, and shorter in March and September (equinoxes) over the equator.
An apparent solar day can be up to 20 seconds shorter or 30 seconds longer than a mean solar day. Long or short days occur in succession, so the difference builds up until mean time is ahead of apparent time by about 14 minutes near February 6, and behind apparent time by about 16 minutes near November 3. The equation of time is this difference, which is cyclical and does not accumulate from year to year. Click here to read more, and see that diagram of these yearly peaks and troughs.

Railway Time click here for more info
In 1847, using telegraph technology, time signals were transmitted across Great Britain to coordinate railroad activities, becoming known as Railway Time.
USA steadily followed 1853-1882 using four time zones, then NSW 1855-1859 (Sydney time), India 1860-1870 (less clearly) with Calcutta Bombay and Madras times, Italy 1866-1867 (Rome time), North Germany 1874 (Berlin time) with all Germany Austria Italy Hungary becoming GMT+1 in 1893, France 1891 (Paris time) noting that in 1911 France delayed Paris Mean Time by 9 minutes 21 seconds to make it equivalent to GMT without mentioning "Greenwich"

, and Ireland (GMT) in 1916. Finally USSR (Moscow) in 1919 — initially GMT+2, but changing to GMT+3 in 1930 with regions further east adjusting their clocks accordingly.

The Greenwich Royal Observatory in London was chosen by the USA (in particular), as the centre for synchronizing world time (before Daylight Saving emerged). On 1 November 1884, Greenwich Mean Time was adopted universally at the International Meridian Conference in Washington, DC with the line at the Observatory bisecting East and West, becoming 0 degrees longitude. An issue involving the Date Line had been The Philippines when their Spanish Governor in 1844 skipped a day and Monday December 30 1844 was followed by Wednesday January 1 1845. And so the International Date Line was drawn up in the Pacific Ocean and 24 time zones were created.

In 1895 Simon Newcomb published his "Tables of the Sun" covering observations 1750-1892. It adjusted the solar year to 365.2422 days.

Originally, astronomers measured a GMT day from noon to noon, while for the general public it started at midnight. In 1928, the name Universal Time or UT was introduced to denote GMT as counted from midnight. And rather than observe the position of the Sun, astronomers found greater accuracy by observing stars as they crossed the meridian each day.

Today, while noting Universal Time (UT1) is tied directly to Earth's rotation angle, Coordinated Universal Time (UTC) is working with an atomic timescale based on Newcomb's tables, a weighted average (in nanoseconds) of 450 atomic clocks worldwide, occasionally adjusted with an intercalary leap second to remain within 0.9 seconds of UT1.

In 1972 the day length in earth's rotation averaged 3 additional milliseconds, dropping to 1 millisecond in 2016 and -1 millisecond in 2021. The decision to add (or subtract) a leap second in UTC comes from the International Earth Rotation Service (IERS) in Paris working with the International Telecommunications Union (ITU)...

In 1955 the BIH (Bureau International de l'Heure) adopted a proposal by William Markowitz, effective January 1, 1956, dividing UT into UT0 (GMT, Pendulum Clock), UT1 (UT0 with small stellar angular differences), and UT2 (UT1 with small seasonal fluctuations). From 1956 to 1972, UT2 was the international standard recommended for radio broadcasting.

UTC Starting in 1956, WWV in Colorado USA broadcast an atomic clock signal stepped by 20 ms increments to bring it into agreement with UT1, thus it was never more than 20 ms in error. By 1960, the Royal Greenwich Observatory, the US Naval Observatory, and the UK National Physical Laboratory coordinated their radio broadcasts so that time steps and frequency changes were coordinated, and the resulting time scale was informally referred to as "Coordinated Universal Time". In 1963, WWV ceased issuing corrections to UT2.

In April 1967, WWV stopped using the local time of the transmitter site (Eastern Time until 1966, and Mountain Time afterwards) and switched to broadcasting Greenwich Mean Time (GMT). The same year the ITU based in Geneva formally established UTC (Co-ordinated Universal Time) as their official time standard, overseen from Paris France by the BIPM (Bureau of International Weights and Measures). On January 1 1972, UTC was defined to follow UT1 within 0.9 seconds with UT2 subsequently becoming irrelevant. In 1974 WWV switched GMT format name to UTC.

Currently UTC deviates from TAI (International Atomic Time fixed on 1 January 1958) by a number of whole seconds. As of 1 January 2017, when another leap second was put into effect, UTC is currently exactly 37 seconds behind TAI. The 37 seconds result from the initial difference of 10 seconds at the start of 1972, plus 27 leap seconds in UTC since 1972.

GPS. At midnight on 5 January 1980 –19 seconds behind TAI– the US Dept of Defense initiated Global Positioning System time. It now uses four GPS satellites at any given moment continuously broadcasting to a GPS receiver on earth a Navigation Message of 37,500 bits of clock and position data. At a rate of 50 bits-per-second this message takes 12½ minutes on a completely cold start. Currently GPS is ahead of UTC by 18 seconds.

Today GMT (always identical with UT / UTC) is the basis for civil time in the United Kingdom, including the BBC World Service, the Royal Navy, and the Met Office. And others, particularly in Arab countries such as the Middle East Broadcasting Centre.

Click here for a live UTC clock, started at midnight December 31st 1899 (called January 0, 1900).

TimeZones Worldwide Sunday 20th July 2025 using AEST

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TimeZones Worldwide Sunday 29th November 2020