Articles About Timekeeping (40)

AM and PM Explained

How does the 12-hour clock system work? Is midnight 12 am or 12 pm?

Commuters and tourists in Canary Wharf, London, United Kingdom.

Daylight Saving Time (DST)

Why do many countries set the clocks back and forth an hour twice a year?

UTC: 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.

Parents playing with their young daughter in the evening sun

History of DST

History of Daylight Saving Time (DST). Why was Daylight Saving Time created, who invented it, and what is the point of having a time change every year?

Illustration of fingers changing the time on a wristwatch.

1 Hour Ahead or Back?

Which way do the clocks go? "Spring forward or fall back" is just one way to remember which way set the clocks for Daylight Saving Time (DST).

UTC & GMT – Same Difference?

Greenwich Mean Time (GMT) is often interchanged or confused with Coordinated Universal Time (UTC). But GMT is a time zone and UTC is a time standard.

Highway with car lights leading to Atlanta at twilight.

The Never-Ending DST Debate

Daylight Saving Time (DST) has been used for more than But do its pros really outweigh its cons?

GMT or BST in the UK?

The UK is not on Greenwich Mean Time (GMT) all year. During Daylight Saving Time the correct time zone is British Summer Time (BST).

Fifth Avenue Clock with the Flatiron Building at the background in New York City.

What Is Standard Time?

Standard time is also known as winter time or normal time in some countries.

A child in a red jumper leaning their head in their hand and trying to eat a bowl of cereal.

Transition to DST

When Daylight Saving Time (DST) starts, clocks go forward; when it ends, clocks go back. So what is Daylight Saving? Do we gain an hour or lose an hour?

Miller map projection of the world with the imaginary International Date Line (IDL) drawn down the image. The Midway Islands, Kiribati, Tonga, and Chatham Islands are labelled on the map.

What Is the International Date Line?

The International Date Line goes through the Pacific Ocean at about 180 degrees longitude. Where is the International Date Line? See it on a time zone map.

Illustration of two people from different sides of the world, talking on the phone.

What Is a Time Zone?

A time zone refers to any region where the same standard time is kept. Interactive Time Zone Map for current time zones worldwide.

International Atomic Time

International Atomic Time (TAI) is one of the main components of Coordinated Universal Time (UTC), the time scale used to determine local times around the world.

Daylight Savings or Saving?

“Daylight Saving Time” is the correct term, but “Daylight Savings Time” with an “s” is at least as commonly used.

Astronomical clock in Prague, the Czech Republic.

When Did DST Start in Europe?

Daylight Saving Time (DST) was first adopted in Europe by Germany in 1916.

Looking up at the Sun framed by buildings.

What Is Solar Noon?

Noon is at 12 o'clock but solar noon can be at a different time. What's the difference between the two?

Why Do We Have Time Zones?

The expansion of transport and communication during the 19th century created a need for a unified time-keeping system, and time zones were introduced.

Equation of Time

Time measured by the Sun is not the same as time measured by our clocks. The difference between the two is called the equation of time.

Closeup shot of the Elizabeth tower housing the Big Ben bell in London

Summer Time or DST?

Summer time is often used to describe Daylight Saving Time (DST), especially the United Kingdom. Winter time instead of standard time is also a common variation.

Radio telescopes of the Very Large Array in New Mexico, USA.

What Is Universal Time?

Universal Time is a solar time standard used in astronomy and to define time on Earth. How is it measured? What's the difference between UT, UT0, UT1, UT2, and UTC?

How Long Is a Day on Earth?

In terms of solar time, most days are a little longer than 24 hours. Exact day length for today and yesterday.

Why UTC – not CUT?

The abbreviation for Coordinated Universal Time is UTC. Why is it not abbreviated as “CUT” or “TUC”?

Internet Time

What is Internet Time, and how to convert between the time zones and beats.

Interactive Timezone Map from timeanddate.com

What Is a Time Zone?

The term time zone can be used to describe several different things.

How Does an Atomic Clock Work?

Atomic clocks are among the most accurate timekeepers in human history. Just how precise are they and how do they work?

A sundial with the Sun casting a shadow.

What Is Local Mean Time?

Use a sundial to determine the time of day and you will probably be off by a few minutes. That's where LMT comes into play. How is it measured and what is it used for?

What Are Leap Seconds?

Every now and then a leap second is added to Coordinated Universal Time (UTC) in order to synchronize clocks worldwide with the Earth's rotation.

Timekeeping by Stars

Out and about at night without a watch or a smartphone readily available? If you are in the Northern Hemisphere use the Big Dipper and the North Star to tell the time.

Earth in space. Elements of this image furnished by NASA.

The Future of Leap Seconds

Organizations and individuals have for many years argued about abolishing leap seconds for good.

Alternative Time Systems

Astronomers use other measures of time, such as Terrestrial Time (TT), which is 32.184 seconds ahead of International Atomic Time (TAI).

Length of Tropical Years 1900–2100

Exact length down to the second of tropical years ranging from 1900 to 2100. It is also known as a solar year, astronomical year, or equinoctial year.

Long-exposure shot of a fast-spinning globe.

Earth Is Speeding Up

Our home planet has been spinning unusually fast lately. 2020 had some of the shortest days on record.

The Earth Keeps Good Time

While we are constantly trying to build a better clock, it’s easy to forget there’s a very accurate timekeeper right under our feet: planet Earth.

A C17 Globemaster military cargo plane takes off from a dusty airstrip, with rugged mountains in the background.

Military Time: The 24-Hour Clock

Military Time uses a 24-hour time format and the NATO alphabet. How does it work? Conversion chart: What is 1500 Zulu Time in 12-hour AM/PM format?

Long exposure of movement of stars, caused by Earth’s rotation

Will a Negative Leap Second Be Needed Soon?

Our planet is currently spinning, on average, ahead of schedule. This might lead to a negative leap second.

Clock with snow on it during the winter time.

Winter Time Is the Same as Standard Time

Winter time is often used to describe the time of year when a country is not on Daylight Saving Time (DST). It is officially known as standard time.

Equation of Time

Why doesn't the year's earliest sunset occur on the winter solstice, even though it is the shortest day of the year in terms of daylight?

View of Earth taken by European Space Agency (ESA) astronauts.

Earth Slows Down, but Remains Unpredictable

Earth rotation update, April 2024: following a recent run of the shortest days ever recorded by atomic clocks, Earth’s rotational speed may be slowing.

A street in Versailles, a suburb of the French capital Paris.

The End of the Leap Second?

A resolution passed on November 18, 2022, means the world may be moving closer to getting rid of leap of seconds.

A view of the Earth from 9000 meters (30,000 feet)

Earth Sets New Record for Shortest Day

On June 29, 2022, Earth recorded its shortest day since scientists began using atomic clocks to measure its rotational speed.