Before the invention of clocks, people marked the time of day with apparent solar time (also called “true” solar time) – for example, the time on a sundial – which was typically different for every settlement.
When well-regulated mechanical clocks became widespread in the early 19th century, each city began to use some local mean solar time. Apparent and mean solar time can differ by up to around 15 minutes (as described by the equation of time) due to the non-circular shape of the Earth’s orbit around the sun and the tilt of the Earth’s axis. Mean solar time has days of equal length, and the difference between the averages is zero after a year.
Greenwich Mean Time (GMT) was established in 1675 when the Royal Observatory was built to aid mariners in determining longitude at sea, providing a standard reference time when each city in England kept a different local time.
Worldwide time zones
Although the first person to propose a worldwide system of time zones was Italian mathematician Quirico Filopanti in his book Miranda! Published in 1858, his idea was unknown outside the pages of his book until long after his death, so it did not influence the adoption of time zones during the 19th century. He proposed 24 hourly time zones, which he called “longitudinal days”, the first centered on the meridian of Rome. He also proposed a universal time to be used in astronomy and telegraphy.
Scottish-born Canadian Sir Sandford Fleming proposed a worldwide system of time zones in 1879. He advocated his system at several international conferences and thus is widely credited with their invention. In 1876, his first proposal was for a global 24-hour clock, conceptually located at the center of the Earth and not linked to any surface meridian. In 1879 he specified that his universal day would begin at the anti-meridian of Greenwich (180th meridian) while conceding that hourly time zones might have some limited local use. He also proposed his system at the International Meridian Conference in October 1884, but it did not adopt his time zones because they were not within its purview. The conference did adopt a universal day of 24 hours beginning at Greenwich midnight but specified that it “shall not interfere with the use of local or standard time where desirable”.
By about 1900, almost all time on Earth was in standard time zones, only some of which used an hourly offset from GMT. Many applied the time at a local astronomical observatory to an entire country without any reference to GMT. It took many decades before all time on Earth was in the form of time zones referred to as some “standard offset” from GMT/UTC. Most major countries had adopted hourly time zones by 1929. Nepal was the last country to adopt a standard offset, shifting slightly to UTC+5:45 in 1986.
Today, all nations use standard time zones for secular purposes, but they do not all apply the concept as originally conceived. Newfoundland, India, Iran, Afghanistan, Venezuela, Burma, the Marquesas, and parts of Australia use half-hour deviations from standard time. Some nations, such as Nepal, and some provinces, such as the Chatham Islands, use quarter-hour deviations. Some countries, most notably China and India, use a single time zone, even though the extent of their territory far exceeds 15° of longitude. Before 1949, China used five time zones.