What is gmt time

To obtain your local time here in the United States, you need to subtract a certain number of hours from UTC depending on how many time zones you are away from Greenwich England.

The table below shows the standard difference from UTC time to local time. The switch to daylight saving time does not affect UTC. It refers to time on the zero or Greenwich meridian, which is not adjusted to reflect changes either to or from Daylight Saving Time.

However, you need to know what happens during daylight saving time in the United States. In short, the local time is advanced one hour during daylight saving time. As an example, the Eastern Time zone difference from UTC is -4 hours during daylight saving time rather than -5 hours as it is during standard time. At first though, Greenwich time was only really important to astronomers. In Maskelyne introduced the Nautical Almanac as part of the great 18th century quest to determine longitude.

This data enabled navigators to find their position at sea. British mariners started keeping at least one chronometer set to GMT. These two solutions would help pave the way for GMT to become the worldwide time standard a century later. Find out about the longitude problem. Until the midth century, almost every town kept its own local time, defined by the Sun. There were no national or international conventions which set how time should be measured.

This meant there was no standard timings for when the day would begin and end, or what length an hour might be. However, the s and s saw the expansion of the railway and communications networks. This meant the need for an national time standard became imperative.

British railway companies started introducing a single standard time across their networks, designed to make their timetables less confusing. It was mostly Greenwich Mean Time that they used. It officially became 'Railway Time'. There were two main reasons for this. The first was that the USA had already chosen Greenwich as the basis for its own national time zone system. As the reference for GMT, the Prime Meridian at Greenwich therefore became the centre of world time and the basis for the global system of time zones.

Therefore this also became the start of the Universal Day. The meridian line is marked by the cross-hairs in the Airy Transit Circle eyepiece. Find out more about the Airy Transit Circle. The Shepherd gate clock can be seen at the gates to the Royal Observatory. It was the first clock ever to show Greenwich Mean Time directly to the public.

It is a 'slave' clock, connected to the Shepherd master clock which was installed at the Royal Observatory in From that time until , the Shepherd master clock was the heart of Britain's time system. Its time was sent by telegraph wires to London, Edinburgh, Glasgow, Dublin, Belfast and many other cities. By , time signals were also sent from the clock to Harvard University in Cambridge, Massachusetts via the new transatlantic submarine cable.

In terms of the distribution of accurate time into everyday life, it is one of the most important clocks ever made. The first thing you notice about the clock is that it has 24 hours on its face rather than the usual That means at 12 noon the hour hand is pointing straight down rather than straight up.

The clock originally indicated astronomical time, in which the counting of the 24 hours of each day starts at noon. The clock was changed in the 20th century to indicate Greenwich Mean Time, in which the counting of the 24 hours of each day starts at midnight. Learn the story of Greenwich Mean Time in our publications.