Time Terminologies: GMT and UTC

Although time terminologies are often spoken and used but still many of us do not have proper understanding of terms like GMT and UTC. Let us have a basic review of these terms.

GMT stands for Greenwich Mean Time (also called Meridian Time or Zulu Time) and it is a time standard since 1884. It was chosen because the Prime Meridian (the line of 0° Longitude) runs through Greenwich. Greenwich was a royal park and palace on a hill to the south of the River Thames east of London. Thus, Greenwich Mean Time (GMT) is the mean solar time at the Royal Observatory in Greenwich, London.

UTC is abbreviated from Coordinated Universal Time and it is The World’s Time Standard. Coordinated Universal Time (UTC) is the basis for civil time today. This 24-hour time standard is kept using highly precise atomic clocks combined with the Earth’s rotation.

UTC is the time standard commonly used across the world. The world’s timing centers have agreed to keep their time scales closely synchronized – or coordinated – therefore the name Coordinated Universal Time.

Two components are used to determine UTC:

• International Atomic Time (TAI): A time scale that combines the output of some 400 highly precise atomic clocks worldwide, and provides the exact speed for our clocks to tick.
• Universal Time (UT1), also known as astronomical time or solar time, refers to the Earth’s rotation. It is used to compare the pace provided by TAI with the actual length of a day on Earth.

GMT vs UTC

GMT is often interchanged or confused with UTC. But GMT is a time zone and UTC is a time standard.

Although GMT and UTC share the same current time in practice, there is a basic difference between the two:

• GMT is a time zone officially used in some European and African countries. The time can be displayed using both the 24-hour format (0 – 24) or the 12-hour format (1 – 12 am/pm).
• UTC is not a time zone, but a time standard that is the basis for civil time and time zones worldwide. This means that no country or territory officially uses UTC as a local time.

UTC does not changes with seasons. GMT was replaced with UTC as reference time scale on 1 January 1972. UTC or atomic time that include leap seconds, guaranteed to always be within 0.9 seconds of GMT.

“Time Zone refers to any region where the same standard time is kept. It is one of more than 24 divisions of the earth, based on sections 7.5 degrees east and west of every 15 degree increment of longitude. There are 24 time zones (360 degrees divided by 15 degrees is 24) plus several offset time zones. Time zones allow the time to follow the rotation of the earth, providing a noon time at the approximate zenith of the sun in the sky of a given location.”

The local time within a time zone is defined by its offset (difference) from UTC, the world’s time standard. This offset is expressed as either UTC- or UTC+ and the number of hours and minutes.

UTC, GMT and Daylight Saving Time

“Daylight Saving Time (DST) is the practice of setting the clocks forward one hour from standard time during the summer months, and back again in the fall, in order to make better use of natural daylight.”

Neither UTC nor GMT ever change for Daylight Saving Time (DST). However, some of the countries that use GMT switch to different time zones during their DST period.

For example, the United Kingdom is not on GMT all year, it uses British Summer Time (BST), which is one hour ahead of GMT, during the summer months.