Prior to 1972, this time was called Greenwich Mean Time (GMT) but is now referred to as Coordinated Universal Time or Universal Time Coordinated (UTC).
Although there was GMT, a committee at the United Nations officially adopted UTC as a standard. This is because it is more accurate than GMT for setting clocks. Coordinated Universal Time and Greenwich Mean Time is used interchangeably; that's why knowing their difference is a must, especially when dealing with time.
There were two main reasons for the choice. The first was the fact that the USA had already chosen Greenwich as the basis for its own national time zone system. The second was that in the late 19th century, 72% of the world's commerce depended on sea-charts which used Greenwich as the Prime Meridian.
GMT is the standard time zone in Ireland and the United Kingdom, including England, Wales, Scotland, and Northern Ireland. All of these countries use DST during part of the year, but under different names. The United Kingdom and its Crown dependencies use British Summer Time during the DST period.
From 1884 until 1972, GMT was the international standard of civil time. Though it has now been replaced by Coordinated Universal Time (UTC), GMT is still the legal time in Britain in the winter, used by the Met Office, Royal Navy and BBC World Service.
GMT time is the current time measured on the Prime Meridian (0 degrees longitude). The Prime Meridian runs through Greenwich, England, hence the name Greenwich Mean Time or GMT.
Greenwich Mean Time (GMT) has no offset from Coordinated Universal Time (UTC). This time zone is in use during standard time in: Europe, Africa, North America, Antarctica.
GMT Seconds make use of the fact there are 86,400 seconds in a day. So, to find the day from GMT Seconds, you would add GMT Seconds / 86400 to 1/1/1980. To find the time, you would take the remainder and divide by 60 to get minutes into the day or 3600 to get hours into the day.
Prior to 1972, this time was called Greenwich Mean Time (GMT) but is now referred to as Coordinated Universal Time or Universal Time Coordinated (UTC). It is a coordinated time scale, maintained by the Bureau International des Poids et Mesures (BIPM).
The United Kingdom uses Greenwich Mean Time or Western European Time (UTC) and British Summer Time or Western European Summer Time (UTC+01:00).
London in GMT Time Zone
London uses Greenwich Mean Time (GMT) during standard time and British Summer Time (BST) during Daylight Saving Time (DST), or summer time.
Greenwich meridian, also called prime meridian, an imaginary line, last established in 1851, that was used to indicate 0° longitude. It passes through Greenwich, a borough of London, and terminates at the North and South poles. Because it indicated 0° longitude, it was also known as the prime meridian.
*IST is ahead of GMT by 5:30 Hours.
Secondly, 72% of the world's commerce in late 19th Century already depended on nautical charts based on Greenwich. As the Greenwich meridian was defined based on the position of the Airy Transit Circle, the prime meridian was also now officially situated there.
GMT World Standard
In 1884 GMT was adopted as the international standard for civil time at the International Meridian Conference in Washington, D.C., USA. It remained the standard until 1972 when it was replaced with Coordinated Universal Time (UTC).
Most of Australia uses three standard time zones, Australian Eastern Standard Time (AEST) which is UTC/GMT+10, Australian Central Standard Time (ACST) which is UTC/GMT + 9 1/2 and Australian Western Time (AWST) which is UTC/GMT +8.
Current time now in Sydney, New South Wales, Australia
Standard Time is 10 hours ahead of Greenwich Mean Time (GMT+10). Exception: YancOwinna (also known as Broken Hill) is in NSW, but adheres to South Australian time, including Daylight Saving Time.
London – Sydney: 9 hours time difference
Moving the clock back one hour from summer to winter time, Sydney has decreased its time difference with London by another hour. Now Brisbane and Sydney have the same time, during their winter months.