When choosing a watch, you often see the term "GMT." To deepen your knowledge of "standard time," which is essential for watches that display time, it's important to understand the meaning of GMT and UTC. This article explains the standard time of each country and how to use a watch equipped with a GMT function.
Learn about GMT
A wristwatch displays the time, but what is the basis for determining this "time" in the first place?
Let's start by looking at GMT and UTC, which are relevant to all clocks.
What is GMT?
"GMT (Greenwich Mean Time)" is one of the international time standards. It refers to the "mean solar time" based on the meridian (north-south line) that passes through the Greenwich Observatory in the UK, and is translated as "Greenwich Mean Time" or "Greenwich Mean Time."
Mean solar time is the average length of a day, 24 hours (86,400 seconds) throughout the year, determined from the relationship between the sun, a fixed star, and the earth's rotation.
The length of a day, as observed from the movement of the fixed stars, varies with the seasons, and the Earth's rotation speed is not constant. GMT was created by averaging these to define a 24-hour period, and was based on the Greenwich Observatory.
Since the location of the Greenwich Observatory is gradually changing due to continental plate movement, the current "Prime Meridian (longitude 0 degrees 0 minutes 0 seconds)" has been corrected to the "IERS Reference Meridian."
What is UTC?
"UTC (Coordinated Universal Time)" is a reference time that has been adopted as the world's "standard time" in place of GMT, and is the result of synchronizing international atomic time (TAI: temps atomique international), which is derived from cesium atomic clocks, with one of the multiple universal time standards (UT1: Universal Time 1).
UTC stands for "Coordinated Universal Time," and since the word order varies depending on the language, we will use the standard spelling of UTC.
Mean solar time varies depending on where on Earth it is observed. UT1 corrects this error, ensuring that the time is the same everywhere in the world, but it is inevitable that the length of a day (LOD: Length of Day) will fluctuate slightly from 86,400 seconds depending on the season.
UTC aligns the one-second interval with TAI. Furthermore, the LOD error that occurs in UT1 is prevented from exceeding one second per year, and a one-second "leap second" is inserted about once every three years to standardize it.
Standard time of the country or region
The world's standard time is expressed based on UTC. In the fields of aviation and communications, GMT is also accepted as a synonym, so the notation GMT is not uncommon.
Using Greenwich, England as the base (UTC/GMT), it represents the 26-hour time difference from UTC+14 (the earliest start of the day) in the Line Islands of Kiribati to UTC-12 in Baker Island, USA.
The time difference is one hour for every 15 degrees of longitude. Standard time is divided into "time zones" in UTC or GMT notation and is specified for each country or region.
In Japan
Japan's standard time is called Japan Standard Time (JST) and is written as "JST" in English. The time signal sounds according to JST.
It refers to the time at the 135th meridian east, which passes through the Akashi Municipal Planetarium in Hyogo Prefecture, and is 9 hours ahead of UTC.
Japan Standard Time refers to the time "UTC+9", and the difference between JST and UTC is generally expressed as "12:00:00 (UTC+0900)". The difference from GMT is also expressed as "GMT+0900".
The following countries and regions have the same standard time as JST.
- KST: Korea Standard Time
- YAKT: Yakutsk Time, Russia
- WIT: Indonesian Eastern Standard Time
- PWT: Palau Time
- TLT: East Timor Time
- PYT: Pyongyang Time
In the United States
The United States uses multiple time zones because the country is long from east to west and has many enclaves.
- UTC+10: ChST (Chamorro Standard Time), Northern Mariana Islands, Guam
- UTC-4: AST (Atlantic Standard Time). U.S. Virgin Islands, Puerto Rico
- UTC-5: EST (Eastern Standard Time). Washington, New York, Boston, etc.
- UTC-6: CST (Central Standard Time), Chicago, Dallas, Houston, etc.
- UTC-7: MST (Mountain Standard Time), Denver, Salt Lake City, etc.
- UTC-8: PST (Pacific Standard Time), Los Angeles, San Francisco, etc.
- UTC-9: AKST (Alaska Standard Time). Anchorage
- UTC-10: HAST (Hawaii Aleutian Standard Time). Oahu, Hawaii Island, etc.
- UTC-11: SST (Samoa Standard Time). Manu'a Islands, etc.
In Europe
European Standard Time defines one time zone per country, excluding exclaves.
- UTC+2: EET (Eastern European Time) in 9 countries including Finland, Ukraine, and Greece
- UTC+1: CET (Central European Time). Most European countries, including France, Germany, Italy, and Spain. Switzerland, the home of mechanical watches, is also included here.
- UTC+0: WET (Western European Time), UK, Portugal, Ireland, Iceland, etc.
In the UK, it is traditionally written as GMT, and summer time as British Summer Time (BST, UTC+1). In other time zones, summer time is written as EEST for EET and CEST for CET.
In the United States, DT is used to mean Daylight Time, EST is written as EDT, and PST is written as PDT.
