1. What Is the International Date Line (IDL)?

The International Date Line is located halfway around the world from the prime meridian (0° longitude) or about 180° east (or west) of Greenwich, London, UK, the reference point of time zones. It is also known as the line of demarcation.

The Dateline Is Not Straight
The dateline runs from the North Pole to the South Pole and marks the divide between the Western and Eastern Hemisphere. It is not straight but zigzags to avoid political and country borders and to not cut some countries in half. On timeanddate.com's Time Zone Map, the IDL is shown as a black line (see image).

What Happens When You Cross the Dateline?

When you cross the International Date Line from west to east, you subtract a day, and if you cross the line from east to west, you add a day.

Depending on which time zone the country follows, the time difference on either side of the line is not always 24 hours. For example, if you travel the 1061 kilometers (659 miles) across the dateline from Baker Island to Tokelau you have to add 25 hours, or 1 day and 1 hour.

Three Dates at the Same Time

Every day between 100 and 11:59 UTC, three different dates on the calendar are in use at the same time on Earth. For example, our Time Zone Converter shows:

At 10:30 UTC on May 2, it is
23:30 (11.30 pm) on May 1 in American Samoa (UTC−11),
06:30 (6:30 am) on May 2 in New York (UTC-5), and
00:30 (0:30 am) on May 3 in Kiritimati (UTC+14).

Changing Sides of the Dateline

The dateline is not defined by international law. Countries are free to choose the date and time zone that they want to observe.

For example, when the Republic of Kiribati gained independence from being a British colony in 1979 some of the islands were on one side of the dateline, and the rest were on the other. They corrected the anomaly in the eastern half of Kiribati by skipping January 1, 1995 and ever since Kiribati has been the first country to enter the New Year.

In 2011, Samoa changed the time zone from UTC-11 to UTC+13 by shifting the dateline to the west and removing December 30, 2011 from the calendar. They did this to facilitate trade with Australia and New Zealand, and Tokelau followed Samoa for the same reasons.

Drawn up in 1884
The 180° meridian was selected as the International Date Line because it mostly runs through the sparsely populated Central Pacific Ocean. It was decided at the International Meridian Conference in 1884 in Washington, D.C. where 26 countries attended.

How Many Time Zones Are There?

How Many Time Zones Are There?

If each time zone were 1 hour apart, there would be 24 in the world. But several time zones have only 30 and 45 minutes offsets, making the total number worldwide much higher.

More Than 24 Time Zones

If each time zone were 1 hour apart, there would be 24 in the world.

However, the International Date Line (IDL) creates 3 more. Also, several time zones are only 30 or 45 minutes apart, increasing the total number of standard time zones even further.

Daylight Saving Time Zones

During Daylight Saving Time (DST) the time zone name and time changes. The words “daylight” or “summer” are then usually included in the name, and the local time is usually set forward 1 hour.

For example, California uses Pacific Daylight Time (PDT) during the DST period with a UTC offset of UTC-7, but Pacific Standard Time (PST) with an offset of UTC-8 during the rest of the year.

Not the Same as Local Time

The term time zone is often confused with local time. For instance, during DST, it is common to say “California and Arizona are now in the same time zone.” However, the correct thing to say would be: “California and Arizona now have the same local time.”

The reason it may be confusing is that California's local time during DST is UTC-7, but the standard time in California is UTC-8. However, Arizona's local time is always UTC-7, because there's no DST in Arizona, and they remain on standard time all year.

Time Zone Borders Vary

Theoretically, each 1-hour time zone is 15 degrees wide, indicating a 1-hour difference in mean solar time. This can be seen as the white and gray stripes on our Time Zone Map and in the image above.

The actual borders on a time zone map have been drawn to correspond with both internal and international borders, and rarely match up exactly with the 15-degree time zone borders.

Some geographically large (wide) countries, like India and China, use only 1 time zone, while it would have been natural to expect several, like in the US or Australia.

Defined by UTC Offset

Every place on Earth is measured in terms of its distance east or west of the prime meridian (0°longitude) in Greenwich, London, United Kingdom. This is also the reference point for Coordinated Universal Time (UTC) with 1 hour per 15 degrees longitude.

You have to divide the longitude, in degrees, by 15 to find the appropriate time zone, in hours. For example:

At 150 degrees west (or 150° W) longitude, the time should be 150 degrees divided by 15 degrees = 10 hours behind UTC, or UTC-10.
At 75 degrees east (or 75° E) longitude, the time would be 75 degrees divided by 15 degrees = 5 hours ahead of UTC, or UTC+5.

Standard Time, Winter Time, Normal Time

Standard time is the local time in a country or region when Daylight Saving Time (DST) is not in use.

Fifth Avenue Clock with the Flatiron Building at the background in New York City.
Standard time is also known as winter time.

Standard time is sometimes referred to as winter time or normal time, while DST may also be called summer time, especially in the UK.

No DST in Most Countries

More than 60% of the countries in the world use standard time all year.

The remaining countries use DST during the summer months, generally setting clocks forward 1 hour from standard time.

DST Period Longest

The periods of standard time and DST are not equally long. The standard time period is often shorter than the DST period. In the US and Canada, for instance, the standard time period is only around 4.5 months. Most European countries are on standard time 5 months of the year and spend 7 months on DST.

Time Zone Name Changes

During DST, the local time zone name and abbreviation changes, usually to include either “daylight” or “summer” in the time zone name.

For example, New York's standard time is Eastern Standard Time (EST), which is 5 hours behind UTC (UTC–5). However, during DST, the city is on Eastern Daylight Time (EDT), which is 4 hours behind UTC (UTC–4).

In day-to-day speak, however, the time zone is often referred to as just Eastern Time, regardless of whether it's standard or Daylight Saving Time.

Time Difference From UTC

The local time within a time zone is defined by its offset (difference) from Coordinated Universal Time (UTC), the world's time standard.

This offset is expressed as either UTC– or UTC+ and the number of hours.

The standard time in a few countries and territories have UTC offsets of 30 or 45 minutes instead of the usual hour.

Countries Switch Time Zones

From time to time, countries decide to change their standard time. Some changes are for political reasons, like in North Korea.

More common reasons are practical, for example facilitating trade with neighboring areas in Canada or to boost tourism in Mexico.

Time Change Without DST

Instead of DST, some locations change their standard time back and forth every year. For example, the Antarctic research station Troll switches from Greenwich Mean Time (GMT) to Central European Summer Time (CEST), which are 2 hours apart.


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.

Before clocks were invented, people kept time using different instruments to observe the Sun’s meridian passing at noon. The earliest time measuring devices we know of are sundials and water clocks.

Chronometers Changed the Game

The pendulum clock was developed during the 17th century. However, these clocks were not sufficiently accurate to be used at sea to determine longitude and for scientific time measurement in the 18th century.

In 1764, the chronometer was invented. Chronometers measured time accurately in spite of motion or varying conditions, and became popular instruments among many merchant mariners during the 19th century.

Clocks Based on the Sun

Even after the chronometer many towns and cities set clocks based on sunsets and sunrises. Dawn and dusk occur at different times, but time differences between distant locations were barely noticeable before the 19th century because of long travel times and the lack of long-distance communications.

The use of local solar time became increasingly awkward as railways and telecommunications improved. Time zones were, therefore, a compromise, relaxing the complex geographic dependence while still allowing local time to be approximate with mean solar time.

19th Century Challenges

American railroads maintained many different time zones during the late 1800s. Each train station set its own clock making it difficult to coordinate train schedules and confusing passengers. Time calculation became a serious problem for people traveling by train (sometimes hundreds of miles in a day), according to the Library of Congress. Every city in the United States used a different time standard, so there were more than 300 local sun-times to choose from. Railroad managers tried to address the problem by establishing 100 railroad time zones, but this was only a partial solution to the problem.

Operators of the new railroad lines needed a new time plan that would offer a uniform train schedule for departures and arrivals. Four standard time zones for the continental United States were introduced on November 18, 1883.

One Prime Meridian

Britain, which already adopted its own standard time system for England, Scotland, and Wales, helped gather international consensus for global time zones in 1884.

Various meridians were used for longitudinal references among different countries before the late 1800s, and the Greenwich Meridian was the most popular of these. The Greenwich Observatory's reputation for the reliability and accuracy in publications of its navigational data was one factor that contributed to the Greenwich Meridian’s popularity. Moreover, the shipping industry would benefit from having just one prime meridian. Many people informally recognized the Greenwich Meridian as the prime meridian before the International Meridian Conference in 1884.

Sir Sandford Fleming was one of the key players in developing a satisfactory worldwide system of keeping time. He advocated the adoption of a standard or mean time and hourly variations from that following established time zones. He also helped convene the International Meridian Conference in 1884, where the international standard time system was adopted.

The International Meridian Conference

The International Meridian Conference in Washington DC, USA, adopted a proposal in October 1884. The proposal stated that the prime meridian for longitude and timekeeping should be one that passes through the center of the transit instrument at the Greenwich Observatory in the United Kingdom (UK). The conference established the Greenwich Meridian as the prime meridian and Greenwich Mean Time (GMT) as the world’s time standard. The international 24-hour time-zone system grew from this, in which all zones referred back to GMT on the prime meridian.

The main factors that favored Greenwich as the site of the prime meridian were:

Britain had more shipping and ships using the Greenwich Meridian than the rest of the world put together (at the time). The British Nautical Almanac started these charts in 1767.
The Greenwich Observatory produced data of the highest quality for a long time.

Time Zones in the 20th Century

Interestingly, many French maps showed zero degrees in Paris for many years despite the International Meridian Conference’s outcomes in 1884. GMT was the universal reference standard – all other times being stated as so many hours ahead or behind it – but the French continued to treat Paris as the prime meridian until 1911. Even so, the French defined their civil time as Paris Mean Time minus 9 minutes and 21 seconds. In other words, this was the same time as GMT.

In 1972, Coordinated Universal Time (UTC) replaced GMT as the world's time standard. France did not formally use UTC as a reference to its standard time zone (UTC+1) until August in 1978.

Standard time, in terms of time zones, was not established in United States law until the Act of March 19, 1918. The act also established daylight saving time in the nation. Daylight saving time was repealed in 1919, but standard time in time zones remained in law, with the Interstate Commerce Commission (ICC) having the authority over time zone boundaries.

Many countries started using hourly time zones by the late 1920s. Many nations today use standard time zones, but some places use 30 or 45 minute deviations from standard time. Some countries such as China use a single time zone even though their territory extends beyond the 15 degrees of longitude.

__________________
“What we need in this country today is more courage and more belief in the things that we have.”- Thomas J. Watson

A Day Is Not Exactly 24 Hours

Exact Day Length* — Tue, 21 Nov 2017
Today's prediction: 24 hours, 0 minutes, 0.0006338 seconds (0.6338 milliseconds)

Yesterday's prediction: 24 hours, 0 minutes, 0.0007564 seconds (0.7564 milliseconds)

At the start of today, UT1 was 0.2582928 seconds ahead of UTC.

Modern timekeeping defines a day as the sum of 24 hours – but that is not quite correct. The Earth's rotation slows down over time. So in terms of solar time, most days are a little longer than 24 hours.

How Long Is Today?
Today is predicted to be 0.6338 ms (milliseconds) or 0.0006338 seconds longer than 24 hours. This is the time it takes Earth to rotate 29.48 cm (11.61 in), as measured at the equator.

This means that today lasts:

24.0000001761 hours or
24 hours and 0.63 ms
On average, a mean solar day in the last 365 days was 1.06 ms over 24 hours, so today's day length is below average. Over this period, 282 days have been longer than today, while 84 have been shorter than today.

If every day was as long as today, a leap second would have to be added every 1577.78 days.

Average Day Lengths & Leap Seconds

In rare cases, a day can also be shorter than 24 hours. The last time this happened was on Mon, 18 Jul 2016 (day was 0.03 ms short). However, the average day exceeds 24 hours. See the table below for yearly average day lengths.

To make up for the additional duration, leap seconds are added to our clocks from time to time.

Why Are the Days Getting Longer?

The speed of the Earth's rotation decreases over time, but it also varies from day to day. One of the main factors are the celestial bodies surrounding us. For example, the Moon's gravitational pull causes tides and changes the Earth's shape, ultimately resulting in a lower rotational speed. The distance between Earth and Moon changes constantly, which makes for daily variations in the speed our planet rotates around its own axis.

__________________
“What we need in this country today is more courage and more belief in the things that we have.”- Thomas J. Watson

What Do AM and PM Stand For?

Some countries, including the US, Canada, and Australia, use the 12-hour clock format including am and pm. What do these abbreviations mean? Is midnight am or pm?

Two 12-Hour Periods
The 12-hour system divides the 24 hours of a day into two periods lasting 12 hours each. The first 12-hour period is designated as am. It runs from midnight to noon. The second period, marked pm, covers the 12 hours from noon to midnight.

The abbreviations am and pm derive from Latin:

AM = Ante meridiem: Before noon
PM = Post meridiem: After noon

Using numbers from 1 to 12, followed by am or pm, the 12-hour clock system identifies all 24 hours of the day. For example, 5 am is early in the morning, and 5 pm is late in the afternoon; 1 am is one hour after midnight, while 11 pm is one hour before midnight.

Ante meridiem is commonly denoted as AM, am, a.m., or A.M.; post meridiem is usually abbreviated PM, pm, p.m., or P.M. Like many other sources, timeanddate.com uses am and pm, but the other variants are equally correct and widely used

Midnight and Noon: AM or PM?

The main weakness of the 12-hour system is a widespread confusion about which abbreviation should be used for noon and midnight: neither moment can logically be identified as before noon (am) or after noon (pm). For example, the moment of midnight occurs precisely 12 hours after noon on the previous day and 12 hours before noon on the following day.

However, most digital clocks and most sources, including timeanddate.com, designate midnight as 12 am and noon as 12 pm. Although the precise moment of noon falls in neither category, the hour succeeding it, from 1201 to 12:59:59, is clearly after noon.

To avoid any confusion when referring to the precise moment of noon or midnight, we recommend using the designations 12 noon and 12 midnight instead.

Midnight Confusion

Another source of confusion is the lack of a date designator in the 12-hour system, making it impossible to logically identify a correct moment in time when only a date and 120 am (midnight) is provided.

Imagine being asked to pick up a friend at the airport at 120 am on April 13. Would you go there at midnight between April 12 and April 13? Or 24 hours later?

One way to overcome this problem is to sacrifice accuracy for clarity. Your friend could ask you to be at the airport at 121 am on April 13 or, if the following midnight is meant, at 11:59 pm on April 13. Alternatively, the 24-hour format could be used. Here, 00 refers to midnight at the beginning of the day while 240 is midnight at the end of the day.

Why Does the Day Have 24 Hours?

The Egyptians are thought to be responsible for dividing the day into 24 equal parts. One account suggests that this custom evolved because the Egyptians commonly used the base 12, counting the 3 joints in their fingers, not including the thumb.

__________________
“What we need in this country today is more courage and more belief in the things that we have.”- Thomas J. Watson

With an error of only 1 second in up to 100 million years, atomic clocks are among the most accurate timekeeping devices in history.

9,192,631,770 Oscillations

Atomic clocks are designed to measure the precise length of a second, the base unit of modern timekeeping. The International System of Units (SI) defines the second as the time it takes a caesium-133 atom in a precisely defined state to oscillate exactly:

9 billion, 192 million, 631 thousand, 770 times.

The official definition provides more detail: “The second is the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium-133 atom. This definition refers to a caesium atom at rest at a temperature of 0 Kelvin.”

Working Principle of Atomic Clocks

In an atomic clock, the natural oscillations of atoms act like the pendulum in a grandfather clock. However, atomic clocks are far more precise than conventional clocks because atomic oscillations have a much higher frequency and are much more stable.

There are many different types of atomic clocks, but they generally share the same basic working principle, which is described below:

Heat, Bundle, and Sort

First, the atoms are heated in an oven and bundled into a beam. Each atom has one of two possible energy states. They are referred to as hyperfine levels, but let's call them state A and state B.

A magnetic field then removes all atoms in state B from the beam, so only atoms in state A remain.

Irridate and Count

The state-A atoms are sent through a resonator where they are subjected to microwave radiation, which triggers some of the atoms to change to state B. Behind the resonator, atoms that are still in state A are removed by a second magnetic field. A detector then counts all atoms that have changed to state B.

Tune and Measure

The percentage of atoms that change their state while passing through the resonator depends on the frequency of the microwave radiation. The more it is in sync with the inherent oscillation frequency of the atoms, the more atoms change their state.

The goal is to perfectly tune the microwave frequency to the oscillation of the atoms, and then measure it. After exactly 9,192,631,770 oscillations, a second has passed.

How Accurate Are Atomic Clocks?

The accuracy of atomic clocks varies and is constantly improving. With an expected error of only 1 second in about 100 million years, the NIST-F1 in Boulder, Colorado, is one of the world's most precise clocks.

It is called a caesium fountain clock where lasers concentrate the atoms into a cloud, cool them down, and then toss them upwards. This method slows the atoms down, allowing for a longer measurement period and a more precise approximation of the natural frequency of the atoms.

Optical Clocks

Scientists are currently developing a device that is even more accurate than the current atomic clocks. The optical atomic clock uses light in the visible spectrum to measure atomic oscillations. The resonance frequency of the light rays is about 50,000 times higher than that of microwave radiation, allowing for a more precise measurement. The expected deviation of the new optical clock is 1 second in 15 billion years.

Why Do We Need Atomic Clocks?

Some 400 atomic clocks around the world contribute to the calculation of International Atomic Time (TAI), one of the time standards used to determine Coordinated Universal Time (UTC) and local times around the world.

Satellite navigation systems like GPS, GLONASS, and Galileo also rely on precise time measurements to calculate positions accurately.

__________________
“What we need in this country today is more courage and more belief in the things that we have.”- Thomas J. Watson