How Time Works

By: Marshall Brain  | 
How did the concept of time originate? Loonger / Getty Images

Time is something that most of us take for granted. Have you ever thought about why, for example, there are 12 months in a year? Why are there 30 days in September? Why are there time zones and what's with daylight-saving time? Why are there 86,400 seconds in a day?

In this article, we'll help to clarify the subject of time. In the same way that many of the traditions surrounding Christmas and Halloween have totally unexpected origins, so do the traditions surrounding clocks and calendars!


Time's Origins

Webster's New World College Dictionary (Fourth Ed.) defines time as:

II. a period or interval. 1: the period between two events or during which something exists, happens or acts; measured or measurable interval

At its core, time is fairly elusive. We can't see it or sense it -- it just happens. Human beings have therefore come up with ways to measure time that are totally arbitrary and also fairly interesting from a historical perspective.


The day is an obvious starting point for time. A day consists of a period of sunlight followed by night. Our bodies are tuned in to this cycle through sleep, so each morning we wake up to a new day. No matter how primitive the culture, the concept of a day arises as an obvious and natural increment.

We use clocks to divide the day into smaller increments. We use calendars to group days together into larger increments. Both of these systems have very interesting origins that we'll find out about in the course of this article.

Measuring Time

The measurement of time covers an incredible range. Here are some common time spans, from the shortest to the longest.

  • 1 picosecond (one-trillionth of a second) - This is about the shortest period of time we can currently measure accurately.
  • 1 nanosecond (one-billionth of a second) - 2 to 4 nanoseconds is the length of time that a typical home computer spends executing one software instruction.
  • 1 microsecond (one-millionth of a second)
  • 1 millisecond (one-thousandth of a second) - This is the typical fastest time for the exposure of film in a normal camera. A picture taken in 1/1,000th of a second will usually stop all human motion.
  • 1 centisecond (one-hundredth of a second) - The length of time it takes for a stroke of lightning to strike
  • 1 decisecond (one-tenth of a second) - A blink of an eye
  • 1 second - An average person's heart beats once each second.
  • 60 seconds - One minute; a long commercial
  • 2 minutes - About as long as a person can hold his or her breath
  • 5 minutes - About as long as anyone can stand waiting at a red light
  • 60 minutes - An hour; about as long as a person can sit in a classroom without glazing over
  • 8 hours - The typical workday in the United States, as well as the typical amount of sleep a person needs every night
  • 24 hours - One day; the amount of time it takes for the planet Earth to rotate one time on its axis
  • 7 days - One week
  • 40 days - About the longest a person can survive without food
  • 365.24 days - One year; the amount of time it takes for the planet Earth to complete one orbit around the sun
  • 10 years - One decade
  • 75 years - The typical life span for a human being
  • 5,000 years - The span of recorded history
  • 50,000 years - The length of time Homo sapiens has existed as a species
  • 65 million years - The length of time dinosaurs have been extinct
  • 200 million years - The length of time mammals have existed
  • 3.5 to 4 billion years - The length of time that life has existed on Earth
  • 4.5 billion years - The age of planet Earth
  • 10 to 15 billion years - The suspected age of the universe since the big bang


How long is a day?

It's the amount of time it takes for the Earth to rotate one time on its axis. But how long does it take the Earth to rotate? That is where things become completely arbitrary. The world has decided to standardize on the following increments:

  • A day consists of two 12-hour periods, for a total of 24 hours.
  • An hour consists of 60 minutes.
  • A minute consists of 60 seconds.
  • Seconds are subdivided on a decimal system into things like "hundredths of a second" or "millionths of a second."

That's a pretty bizarre way to divide a day up. We divide it in half, then divide the halves by twelfths, then divide the twelfths into sixtieths, then divide by 60 again, and then convert to a decimal system for the smallest increments. It's no wonder children have trouble learning how to tell time.


Why are there 24 hours in a day?

No one really knows. However, the tradition goes back a long way. Take, for example, this quote from Encyclopedia Britannica:

The earliest known sundial still preserved is an Egyptian shadow clock of green schist dating at least from the 8th century BC. It consists of a straight base with a raised crosspiece at one end. The base, on which is inscribed a scale of six time divisions, is placed in an east-west direction with the crosspiece at the east end in the morning and the west end in the afternoon. The shadow of the crosspiece on this base indicates the time. Clocks of this kind are still in use in primitive parts of Egypt.

The Babylonians seem to be the ones who started the six fetish, but it is not clear why.

Why are there 60 minutes in an hour and 60 seconds in a minute?

Again, it is unclear. It is known, however, that Egyptians once used a calendar that had 12 30-day months, giving them 360 days. This is believed to be the reason why we now divide circles into 360 degrees. Dividing 360 by 6 gives you 60, and 60 is also a base number in the Babylonian math system.

What do a.m. and p.m. mean?

These abbreviations stand for ante meridiem, before midday, and post meridiem, after midday, and they are a Roman invention. According to Daniel Boorstin in his book The Discoverers, this simple division of the day into two parts was the Romans' first increment of time within a day:

Even at the end of the fourth century B.C., the Romans formally divided their day into only two parts: a.m. and p.m. An assistant to the consul was assigned to notice when the sun crossed the meridian, and to announce it in the Forum, since lawyers had to appear in the courts before noon.

Modern man bases time on the second. A day is defined as 86,400 seconds, and a second is officially defined as 9,192,631,770 oscillations of a cesium-133 atom in an atomic clock.

Time Zones

Everyone on the planet wants the sun to be at its highest point in the sky (crossing the meridian) at noon. If there were just one time zone, this would be impossible because the Earth rotates 15 degrees every hour. The idea behind multiple time zones is to divide the world into 24 15-degree slices and set the clocks accordingly in each zone. All of the people in a given zone set their clocks the same way, and each zone is one hour different from the next.

In the continental United States there are four time zones (click here for a map): Eastern, Central, Mountain and Pacific. When it is noon in the Eastern time zone, it is 11 a.m. in the Central time zone, 10 a.m. in the Mountain time zone and 9 a.m. in the Pacific time zone.


All time zones are measured from a starting point centered at England's Greenwich Observatory. This point is known as the Greenwich Meridian or the Prime Meridian. Time at the Greenwich Meridian is known as Greenwich Mean Time (GMT) or Universal Time. The Eastern time zone in the United States is designated as GMT minus five hours. When it is noon in the Eastern time zone, it is 5 p.m. at the Greenwich Observatory. The International Date Line (IDL) is located on the opposite side of the planet from the Greenwich Observatory.

Why is the Greenwich Observatory such a big deal? A bunch of astronomers declared the Greenwich Observatory to be the prime meridian at an 1884 conference. What's funny is that the observatory moved to Sussex in the 1950s, but the original site remains the prime meridian.

Daylight-saving Time

During World War I, many countries started adjusting their clocks during part of the year. The idea was to try to adjust daylight hours in the summer to more closely match the hours that people are awake. During World War I, the goal was to conserve fuel by lowering the need for artificial light.

The United States and several other countries still use some variation on this system. In the United States, traditionally, daylight-saving time has started on the first Sunday in April and ended on the last Sunday in October. However, the Energy Policy Act of 2005 mandated a change to the observed dates. Starting in 2007 and going forward, DST will now begin at 2 a.m. on the second Sunday of March and will end at 2 a.m. on the first Sunday in November. Here are the start and stop dates through 2015:


To observe DST, clocks are advanced one hour in the spring and moved back one hour in the fall ("spring forward, fall back" is a phrase many people use to remember this). You lose an hour in the spring and get it back in the fall.

During the winter, the United States is on standard time. During the summer, the United States is on daylight-saving time. Even though it's an act of Congress, some states (like Arizona) ignore it and don't have daylight-saving time. They are on standard time all year.

See WebExhibits: Daylight Saving Time to learn more.

The Calendar: Years

As mentioned earlier, the day is an obvious unit of time for people. But what about weeks, months and years?

Years are fairly straightforward. Man created the concept of a year because seasons repeat on a yearly basis. The ability to predict seasons is essential to life if you are planting crops or trying to prepare for winter. Most plants sprout and bear fruit on a yearly schedule, so it's a natural increment.


A year is defined as the amount of time it takes for the Earth to orbit the sun one time. It takes about 365 days to do that. If you measure the exact amount of time it takes for the Earth to orbit the sun, the number is actually 365.242199 days (according to Encyclopedia Britannica). By adding one extra day to every fourth year, we get an average of 365.25 days per year, which is fairly close to the actual number. This is why we have leap years that are one day longer than normal years.

To get even closer to the actual number, every 100 years is not a leap year, but every 400 years is a leap year. Putting all of these rules together, you can see that a year is a leap year not only if it is divisible by 4 -- it also has to be divisible by 400 if it is a centurial year. So 1700, 1800 and 1900 were not leap years, but 2000 was. That brings the average length of the year to 365.2425 days, which is even closer to the actual number.

The problem with the concept of a year is that it is hard to determine the exact length of a year unless your society has fairly good astronomers. Many cultures that lacked astronomers relied on the cycles of the moon instead. A moon cycle lasts approximately 29.5 days (29.530588 days is the exact number), and it is easy for almost anyone to track the moon's cycle simply by looking at the sky every night.

The Calendar: Months

The moon is where the concept of a month comes from. Many cultures used months whose lengths were 29 or 30 days (or some alternation) to chop up a year into increments. The main problem with this sort of system is that moon cycles, at 29.5 days, do not divide evenly into the 365.25 days of a year.

When you look at the modern calendar, the months are extremely confusing. One has 28 or 29 days, some have 30 days and the rest have 31 days. According to the "World Book Encyclopedia," here is how we got such a funny calendar:


  • The Romans started with a 10-month calendar in 738 B.C., borrowing from the Greeks. The months in the original Roman calendar were Martius, Aprilis, Maius, Junius, Quintilis, Sextilis, September, October, November and December. The names Quintilis through December come from the Roman names for five, six, seven, eight, nine and 10. This calendar left 60 or so days unaccounted for.
  • The months Januarius and Februarius were later added to the end of the year to account for the 60 spare days.
  • In 46 B.C., Julius Caesar changed the calendar. Ignoring the moon but keeping the existing 12 month's names, the year was divided into 12 months having 30 or 31 days, except Februarius at the end with 29 days. Every fourth year, Februarius gained an extra day. Later, he decided to make Januarius the first month instead of Martius, making Februarius the second month, which explains why leap day is at such a funny point in the year.
  • After Julius' untimely death, the Romans renamed Quintilis in his honor, hence July.
  • Similarly, Sextilis was renamed to honor Augustus, hence August. Augustus also moved a day from Februarius to Augustus so that it would have the same number of days as Julius.

This little history explains why we have 12 months, why the months have the number of days they have, why leap day falls at such an odd time and why the months have such funny names.

What about weeks? Days, months and years all have a natural basis, but weeks do not. They come straight out of the Bible:

Remember the sabbath day, to keep it holy. Six days shalt though labor, and do all thy work but the seventh day is the sabbath of the Lord thy God. (Exodus 20:8)

This fourth commandment, of course, echoes the creation story in Genesis.

The Romans gave names to the days of the week based on the sun, the moon and the names of the five planets known to the Romans:

  • Sun
  • Moon
  • Mars
  • Mercury
  • Jupiter
  • Venus
  • Saturn

These names actually carried through to European languages fairly closely, and in English the names of Sunday, Monday and Saturday made it straight through. The other four names in English were replaced with names from Anglo-Saxon gods. According to Encyclopedia Britannica:

Tuesday comes from Tiu, or Tiw, the Anglo-Saxon name for Tyr, the Norse god of war. Tyr was one of the sons of Odin, or Woden, the supreme deity after whom Wednesday was named. Similarly, Thursday originates from Thor's-day, named in honour of Thor, the god of thunder. Friday was derived from Frigg's-day, Frigg, the wife of Odin, representing love and beauty, in Norse mythology.

B.C. and A.D.

In the modern calendar, we label all years with B.C. (before Christ) or A.D. (anno domini, or "in the year of our lord"). There is no "zero" year -- in this system, the year Christ was born is 1 A.D., and the year preceding it is 1 B.C.

This practice was first suggested in the sixth century A.D., and was adopted by the pope of that time. It took quite a while for it to become a worldwide standard, however. Russia and Turkey, for example, did not convert to the modern calendar and year scheme until the 20th century.


One interesting side note: Because of a variety of changes and adjustments made to the calendar during the middle ages, it turns out that Jesus was most likely born in what we now think of as 6 B.C., and likely lived until 30 A.D.

Besides B.C. and A.D., many people use B.C.E. (for "before common era") and C.E. (for "common era"). These correspond to the same dates as B.C. and A.D., but is a secular way of saying it. In fact, EarthSky reports that Jewish academics have used B.C.E./C.E. for over 100 years.

Originally Published: Apr 1, 2000