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UTC Time explained, formerly GMT. And what is the International Date Line?

UTC Time, Coordinated Universal Time

UTC – Coordinated Universal Time

As the name suggests it acts as the ‘world-time’, a base to which all time zones and therefore time data refers or can refer.

Formerly known as GMT, which means Greenwich Mean Time.
The Royal Observatory was built in Greenwich many years ago to find a way to determine the longitude at sea, which was particularly important for the British Sea Fleet. At the International Meridian Conference in Washington D.C. in 1884 25 countries agreed on accepting that the Prime Meridian, Longitude 0 degree and therefore base for all measurements, should be the meridian line through Greenwich. A meridian is a line running from north to south, dividing the earth into east and west from longitude Zero. All positions east of the Prime Meridian are measured in plus (+) degrees, western positions in minus (-) degrees from the Prime Meridian.

The British Royal Navy of the 19th century used the time at longitude Zero, meaning the time in Greenwich, as time base. GMT was born and became international standard, the so-called ‘world time’. This made it possible that an international date line could be established and international communication could refer to one certain time which reduced the confusion with time and calendar date.

Additionally to the date line there were 24 time zones created.
In 1970 the International Telecommunication Union (ITU) refined GMT time and agreed on a new name: UTC (Universal Coordinated Time). Difference between UTC and GMT is that UTC is measured from midnight on while GMT is based on a noon measurement, meaning a day started at noon! Today the abbreviation GMT is used synonymous with UTC, but GMT is depreciated.
Furthermore the precision of time measurement has been improved by use of several atomic clocks around the world. Satellites distribute UTC (time) as well and so the GPS (system) can utilize UTC.

Military and aviation personal spell a certain time e.g. like: 1130 Zulu. In written it is 1130Z.
Zulu comes from the International Aviation and Military Phonetic Alphabet, which was invented to avoid misunderstanding when spelling.

There exist 24 time zones in the world and every one has got a letter assigned to it.
‘Z’ refers to the UTC (former GMT) time, ‘based’ in Greenwich England.
E.g. 1130Z means that it is 11.30 (before noon) in England.
UTC time is also called Zulu-Time.

What is the International Date Line?

UTC Time Zones and International Date Line [graphic composite: travel1000places.com]
UTC Time Zones and International Date Line [graphic composite: travel1000places.com]

The International Date Line serves as the “line of demarcation” between two consecutive calendar dates.

The International Date Line defines the boundary between calendar dates.

The International Date Line, established in 1884, passes through the mid-Pacific Ocean and roughly follows a 180 degrees longitude north-south line on the Earth. It is located halfway round the world from the prime meridian—the zero degrees longitude established in Greenwich, England, in 1852.

The International Date Line functions as a “line of demarcation” separating two consecutive calendar dates. When you cross the date line, you become a time traveler of sorts! Cross to the west and it’s one day later; cross back and you’ve “gone back in time.”

Despite its name, the International Date Line has no legal international status and countries are free to choose the dates that they observe. While the date line generally runs north to south from pole to pole, it zigzags around political borders such as eastern Russia and Alaska’s Aleutian Islands.

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