The Active Hours display lets you instantly see when your active hours overlap with someone else's, so you can find the best time to call without waking them up (and other uses). Set the person's name, their time zone and then drag and stretch the time bar to their waking hours. Their hours are displayed inside the bar. You can also use this for work times and stock market hours. The vertical, red bar is the NOW bar.
Let's you see what time it is in other time zones and by changing any of the clocks, lets you see what the time would be in other time zones even when it's not actually that time. Move the clock to the desired time in one zone, and all the other zones adjust to that would-be time. Most other time zone tools only show you the time zone in other countries. This one lets you predict the time.
Each tool answers a different kind of time question. Here are some examples of what they are designed to do.
Compare people's normal active hours on one shared timeline. Add work, stores and stock market hours too. Immediately see when their hours overlap with yours.
Let's you see what time it is all over the world and by changing any of the clocks, lets you see what the time would be in other time zones. Most other clocks only show you the time zone in other countries. This one let's you do what-if predictions.
Calculate the exact elapsed time between two dates and times. The result is shown as a human-readable duration as well as separate days, hours, minutes, and seconds.
Turn dates and timestamps into a consistent standard representation. It accepts ISO dates, RFC-style dates, browser-recognized date strings, and Unix timestamps. This also accepts time durations, for example, 2d12h34m10s means "2 days, 12 hours, 34 minutes and 10 seconds" you can use it to calculate times like "NOW + 6h30m" for "what time would be 6 hours and 30 minutes from now"
For most of human history, there was no such thing as a time zone.
People knew what time it was by looking at the Sun. When the Sun reached its highest point in the sky, it was approximately noon. Morning was the period after sunrise, evening came after sunset, and the length of an hour changed with the seasons. There was no need to know what time it was hundreds or thousands of miles away because there was little reason to coordinate activities with people that far away.
Then the world became connected.
Railroads allowed people to travel hundreds of miles in a day. Telegraphs allowed messages to cross continents almost instantly. Ships crossed oceans on schedules. Factories needed workers to arrive at particular times. Governments needed standardized schedules. Eventually, the entire planet needed a common system for describing when an event happened.
Time zones were the solution.
Today, the world is divided into broad regions in which clocks are intended to show the same time. But the system is considerably more complicated than simply dividing the Earth into 24 equal slices. Political borders, geography, history, daylight saving time, half-hour and quarter-hour offsets, and international conventions have all shaped the modern map of time.
The reason time zones were necessary begins with a simple fact about the Earth: it rotates.
The Earth makes approximately one rotation every 24 hours. As it rotates, different parts of its surface face the Sun. Consequently, noon occurs at different moments at different longitudes.
If you stand in New York and observe the Sun reaching its highest point in the sky, that happens several hours after the same event occurs in London. If you travel westward, local noon occurs later. If you travel eastward, it occurs earlier.
Before the development of modern transportation and communication, this was not a problem.
A town could simply set its clocks according to its own local solar time.
For example, suppose the Sun reached its highest point over a particular town at 12:00. That town could call that moment noon. A town 20 miles to the east would experience solar noon a few minutes earlier, while a town 20 miles to the west would experience it a few minutes later.
Nobody particularly cared.
If a farmer said he would meet someone at noon, the meeting was taking place in the same town. There was no need to specify whose noon.
The basic relationship between longitude and time is remarkably simple.
The Earth rotates through 360 degrees in approximately 24 hours.
360° ÷ 24 = 15° per hour
The Earth rotates approximately 15 degrees of longitude every hour.
That means that moving 15 degrees east or west corresponds roughly to a one-hour difference in local solar time.
More precisely, one degree of longitude corresponds to about four minutes of time:
1° = approximately 4 minutes
This relationship is the foundation of the modern time-zone system.
If two locations are separated by 30 degrees of longitude, their local solar times differ by approximately two hours.
This is why the idea of dividing the world into approximately 24 one-hour zones makes geographical sense.
But the actual time zones on a modern map do not follow longitude lines exactly.
To establish a global system, humanity first needed to agree on where longitude zero would be.
That line became the Prime Meridian, passing through Greenwich, England.
The choice of Greenwich was strongly influenced by Britain's enormous maritime and scientific importance during the 18th and 19th centuries. British navigational charts and the work of the Royal Observatory at Greenwich had already made the Greenwich meridian important in international navigation.
In 1884, representatives from 25 countries met at the International Meridian Conference in Washington, D.C.
The conference selected the meridian passing through Greenwich as the international prime meridian.
This established the reference point from which longitude could be measured:
The modern system of Universal Time and time zones ultimately grew from this convention.
The time associated with the Greenwich meridian became known as Greenwich Mean Time, or GMT.
The word "mean" is important.
The apparent solar day is not exactly the same length throughout the year because the Earth's orbit around the Sun is not perfectly circular and because the Earth's rotational axis is tilted. Consequently, the apparent position of the Sun can run ahead of or behind a perfectly uniform clock.
Astronomers therefore developed the concept of mean solar time, which smooths out these variations.
GMT became an important international reference for navigation, astronomy, railways, telecommunications, and eventually aviation.
Today, the technically preferred international reference is Coordinated Universal Time, or UTC.
UTC is based on atomic clocks, with adjustments made to keep it reasonably close to Earth's rotation.
GMT and UTC are often treated as equivalent for everyday purposes, but they are not technically the same thing.
The invention of railroads created one of the strongest practical arguments for standardized time.
Imagine a railroad running hundreds of miles from east to west.
Under local solar time, every town along the railway could have a slightly different time.
A train schedule might therefore look something like this:
| Station | Local Time |
|---|---|
| Town A | 10:00 |
| Town B | 10:07 |
| Town C | 10:14 |
| Town D | 10:22 |
| Town E | 10:29 |
Those differences are perfectly logical astronomically.
They are also terrible for a railroad timetable.
A passenger traveling across the country would have to understand that every town used a slightly different clock. Railroad companies therefore began creating standardized "railroad time" systems.
In the United States, railroads adopted standardized time zones in 1883.
This became known as the Day of Two Noons because different communities adopted the new standardized times at different moments, and local noon could suddenly occur at something other than exactly 12:00 on the local clock.
The federal government did not immediately impose a single nationwide time-zone system. The railroads effectively established one because they needed it to operate reliably.
The railroad system eventually evolved into the familiar U.S. time zones:
The boundaries were not simply straight lines every 15 degrees of longitude.
Instead, they generally followed practical considerations such as state boundaries, transportation networks, and local preferences.
This illustrates one of the most important facts about time zones:
Time zones are partly astronomical and partly political.
If time zones were determined strictly by longitude, their boundaries would form relatively simple north-south bands.
Instead, they frequently bend and zigzag.
Consider two cities separated by a political border.
Astronomically, they might have almost exactly the same solar time. It would therefore be perfectly reasonable for them to share a time zone.
But if the two countries use different standard times for political or economic reasons, the border becomes a time-zone boundary.
The reverse can also happen.
A country can span a huge range of longitude but choose to use a single national time.
China is an excellent example.
China stretches across roughly five geographical time zones, but the entire country officially uses China Standard Time, which is UTC+8.
Consequently, solar noon occurs considerably earlier on the clock in eastern China than it does in western China.
The system works because clock time is a social convention rather than a measurement that must exactly represent the position of the Sun.
There is another problem that appears when you travel around the Earth.
If you continuously move eastward, local time becomes progressively later.
Travel far enough and eventually your clock will have moved through a complete 24 hours.
You have effectively crossed into the next calendar day.
To keep the world's calendar coherent, there must be a place where the date changes.
That is the purpose of the International Date Line.
It generally follows the 180th meridian in the Pacific Ocean, opposite the Prime Meridian.
But, like time-zone boundaries, it is not a perfectly straight line.
It bends around countries and island groups so that communities that belong together can remain on the same calendar date.
Crossing the International Date Line can therefore produce one of the strangest experiences in ordinary travel:
You can travel only a short physical distance but suddenly find that the calendar has advanced or moved backward by a day.
It is tempting to imagine the Earth as 24 neat slices:
15° 30° 45°
| | |
Zone | Zone | Zone |
----------+---------+---------+----
But the real world doesn't work that way.
There are currently many more than 24 civil time offsets.
Most countries use whole-hour offsets such as:
But some use half-hour offsets.
For example:
Others use quarter-hour offsets.
Nepal uses:
UTC+5:45
These unusual offsets demonstrate that the "24 time zones" concept is really an approximation based on the Earth's rotation.
Civil time is whatever a government or other authority establishes as the legal time for its territory.
There is an important technical distinction between a time zone and a UTC offset.
An offset is simply a number:
UTC+7
A time zone is a set of rules describing how a location's civil time relates to UTC over time.
For example:
America/New_York
is a time zone.
At different times of year, it can correspond to:
UTC−5
or:
UTC−4
because of daylight saving time.
Therefore, software should generally store a geographical time-zone identifier rather than simply storing an offset.
If you tell a computer:
New York is UTC−5
you have not completely described New York's civil time.
If you tell it:
America/New_York
you have provided the information needed to determine the appropriate offset for a particular date.
This distinction becomes extremely important when scheduling events.
Daylight saving time adds another layer of complexity.
The basic idea is simple: shift the clock forward during part of the year so that more daylight occurs during the hours when people are normally awake.
For example, a region might change:
2:00 AM → 3:00 AM
in the spring.
Later in the year, it reverses the change:
2:00 AM → 1:00 AM
The result is commonly described as "an extra hour of daylight" in the evening, although the total amount of daylight has obviously not changed. What changes is where that daylight falls relative to the clock.
Daylight saving time has been adopted, abandoned, modified, and reintroduced by different countries at different times.
Some countries use it today.
Others have abolished it.
Some have never used it.
And some have changed their rules multiple times.
This creates one of the reasons time-zone software cannot simply calculate time by adding a fixed number of hours.
Suppose you want to know:
What time was it in New York on July 1, 1940?
You cannot necessarily answer that correctly by simply applying today's UTC offset.
Time-zone rules change.
Countries have changed their standard time, introduced daylight saving time, abolished it, restored it, moved their clocks by unusual amounts, or changed the dates on which seasonal changes occur.
Modern computer systems therefore rely on databases containing historical time-zone rules.
The most important of these is the IANA Time Zone Database, often called the tz database.
It contains identifiers such as:
America/New_York
Europe/London
Asia/Bangkok
Asia/Tokyo
Australia/Sydneyand historical rules describing how those locations have related their civil time to UTC.
This is why a good time-zone calculator can answer questions that would be impossible using a simple table of UTC offsets.
The modern world made time zones vastly more important than they were in the railroad era.
A person in New York can have a video meeting with someone in London, Bangkok, Tokyo, and Sydney.
Everyone needs to know when the meeting occurs for them.
Suppose a meeting is scheduled for:
10:00 AM New York time
At one point in the year, London might be five hours ahead.
Bangkok might be eleven or twelve hours ahead depending on the U.S. daylight-saving period.
Tokyo might be thirteen or fourteen hours ahead.
Sydney can have yet another relationship because Australia has its own daylight-saving rules.
Consequently, saying simply:
"Let's meet at 10."
is nearly meaningless in an international environment.
A useful time specification needs a location or an offset.
For example:
10:00 AM America/New_York
is much more precise.
There are actually several different questions hidden inside the phrase "What time is it?"
You might mean:
These are not necessarily the same question.
For everyday purposes, civil clock time is what matters.
But understanding the distinction explains many of the apparent oddities of time zones.
Imagine two cities separated by 1,000 kilometers from east to west.
The Sun does not reach its highest point over both cities at the same instant.
Nevertheless, their clocks may show exactly the same time.
That is because civil time is designed primarily to coordinate human activity.
The clock is not intended to be a precise astronomical instrument.
This distinction becomes particularly obvious near the eastern and western edges of a large time zone.
Two locations can share the same legal time while having noticeably different relationships between clock time and the Sun.
In other words:
Noon on the clock does not necessarily mean solar noon.
Given the complexity of the system, why not simply use UTC everywhere?
Technically, we could.
There would be significant advantages.
Every computer, airline, bank, company, and person could use the same clock. A meeting would simply be scheduled for:
15:00 UTC
and everyone would convert it if necessary.
In fact, many technical systems effectively do exactly this.
Servers, databases, aviation systems, scientific instruments, and internet protocols frequently use UTC internally.
But using UTC as the only civil time would be awkward for everyday life.
Imagine living in a city where sunrise occurs around 18:00 and sunset around 06:00 according to the official clock.
Nothing physical would be wrong.
The Sun would behave exactly as it always does.
But the clock would be poorly aligned with people's daily routines.
Civil time therefore attempts to strike a balance between astronomical reality and human convenience.
A time zone is essentially a compromise between three things:
Astronomy
The Earth rotates, producing a natural relationship between longitude and solar time.
Geography
People live in places with different longitudes and natural daylight cycles.
Politics and society
Countries, governments, businesses, transportation systems, and communities need common schedules.
The resulting system is neither purely astronomical nor purely political.
It is a practical convention built on top of astronomical reality.
One of the most interesting consequences is that time-zone boundaries can tell you something about politics and economics.
A country may choose a time zone that aligns it with its major trading partners rather than with its geographic position.
A region may change its time zone to simplify transportation or commerce.
A government may decide that an entire country should share one national time.
And neighboring countries can make different decisions even when their geography is nearly identical.
Consequently, the time shown on a clock can sometimes reveal more about the political organization of a region than about the position of the Sun.
Time zones became particularly challenging once computers began scheduling events.
A computer needs to distinguish between:
10:00 AM every day
and:
10:00 AM New York time every day.
Those are not necessarily equivalent.
If the event is scheduled using New York's time zone, its UTC time can change when daylight saving time begins or ends.
For example, a recurring meeting might occur at:
14:00 UTC
during one part of the year and:
15:00 UTC
during another.
From the perspective of the person in New York, however, the meeting remains at:
10:00 AM
That is precisely why calendar programs store both the local time and the applicable time-zone rules.
Today, when we say someone is "in a time zone," we generally mean that their local civil clock follows a particular set of rules.
Those rules determine:
The geographic name attached to a time zone is therefore often more useful than the numerical offset.
For example:
Asia/Bangkok
is much more informative than:
UTC+7
because UTC+7 tells you the current offset but not necessarily the complete set of rules associated with a place.
At first glance, converting time zones appears trivial.
If Bangkok is seven hours ahead of UTC, you might think:
New York time + 12 hours = Bangkok time.
Sometimes that works.
But it doesn't always.
The United States observes daylight saving time, while Thailand does not.
Therefore the difference between New York and Bangkok changes during the year.
The same problem occurs throughout the world.
Even worse, governments sometimes change their time-zone rules with little warning.
A good time-zone conversion system therefore needs access to current and historical time-zone rules rather than merely a table of offsets.
There have been periodic proposals to simplify or eliminate time zones.
One possibility would be to use UTC universally.
Another would be to reduce the number of time zones.
There have also been recurring debates over whether daylight saving time should be abolished or made permanent.
These proposals illustrate an important point:
The underlying astronomical phenomenon is fixed.
The Earth's rotation does not care what humans decide to call noon.
But civil time is a human invention, and therefore it can be changed.
If governments collectively decided tomorrow that a particular country would move its clocks forward by one hour, the Sun would not change at all. Only the labels humans put on moments would change.
Perhaps the most interesting thing about time zones is that they demonstrate the difference between time itself and our measurement of time.
The Earth rotates.
The Sun rises and sets.
Physical events happen in a particular order.
Those are physical phenomena.
But:
10:00 AM
is a human convention.
So is:
Monday
and:
UTC+7
and:
9:00 AM Eastern Time
These conventions allow billions of people to coordinate their activities.
A time zone is therefore not a physical boundary in the world. It is a rule for labeling moments.
That distinction becomes increasingly important as the world becomes more interconnected.
A person can sit in Bangkok and conduct business with someone in New York, attend a meeting hosted in London, monitor a market in Tokyo, and schedule a flight departing from Sydney—all without either person physically traveling anywhere.
The clocks have to travel for us.
Humanity's measurement of time has gone through an extraordinary transformation.
Early societies measured time through the movement of the Sun and stars.
Mechanical clocks provided increasingly precise measurements.
Cities developed their own local times.
Railroads made local time inconvenient.
Telegraphs made synchronized time increasingly necessary.
International conferences established common references.
Atomic clocks eventually made timekeeping extraordinarily precise.
Computers turned time-zone conversion into a software problem.
And the Internet made it possible for people around the planet to coordinate activities almost instantaneously.
The modern time-zone system is the result of all of these developments layered together.
It is simultaneously astronomical, historical, technological, geographical, and political.
When dealing with time across different locations, there are three concepts worth keeping separate:
Local time — what the clock in a particular place says.
UTC offset — how far that local time currently differs from UTC.
Time zone — the geographical and historical rule set that determines the local time and its changing offset.
Once those distinctions are understood, most of the apparent complexity of international time becomes much easier to explain.
The world does not actually contain hundreds of different kinds of time.
There is one continuously progressing physical timeline.
Humanity has simply created a collection of local labels for that timeline so that sunrise, work, meals, transportation, business, and daily life occur at reasonably convenient hours.
And that is ultimately why we have time zones.
The Earth gives us one continuous rotation. Time zones give billions of people a practical way to agree on what to call the moments occurring along it.