UDATE - Unified Date And Time Enhancement

History

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Why is there 24 hours a day?

Why is there 60 minutes in an hour?

Why is there 7 days a week?

Why is there 28,29,30 or 31 days a month?

Why is there 365.25 days a year?

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Short summary:
A Strange History of the Calendar

Full length lecture:
A Brief History of Timekeeping | How Humans Began Telling Time

Why is there 24 hours a day?

Have you ever wondered why the wristwatch strapped to your arm (assuming you still use one), your mobile screen, or any other damn clock in the world shows 12 numbers signifying 12 hours? Why isn’t it some other random number, like 28 or 16? What is the significance of twelve in our perception of time? I mean, if you give it any thought, you can’t deny that our measuring system is pretty weird. Allow me to explain:

24 hours are divided into two parts — a day lasting 12 hours and a night lasting 12 hours
1 hour contains 60 minutes, which also have 60 seconds each.
Each second is then divided into 1000 milliseconds.

Now, that seems like a rather strange way to divide a day. No wonder kids have trouble learning how to tell time! However, as always, like every other thing in the world, there is a reason behind this too.

In today's world, the most widely used numeral system is decimal (base 10), a system that probably originated because it made it easy for humans to count using their fingers. The civilizations that first divided the day into smaller parts, however, used different numeral systems, specifically duodecimal (base 12) and sexagesimal (base 60).

That obviously begs the question — why base 12 and base 60? The reason is very simple, yet quite surprising!

Thanks to documented evidence of the Egyptians' use of sundials, most historians credit them with being the first civilization to divide the day into smaller parts. The first sundials were simply stakes placed in the ground that indicated time by the length and direction of the resulting shadow. As early as 1500 B.C., the Egyptians had developed a more advanced sundial. A T-shaped bar placed in the ground, this instrument was calibrated to divide the interval between sunrise and sunset into 12 parts. This division reflected Egypt's use of the duodecimal system--the importance of the number 12 is typically attributed either to the fact that it equals the number of lunar cycles in a year or the number of finger joints on each hand (three in each of the four fingers, excluding the thumb), making it possible to count to 12 with the thumb. The next-generation sundial likely formed the first representation of what we now call the hour. Although the hours within a given day were approximately equal, their lengths varied during the year, with summer hours being much longer than winter hours.

Yes, believe it or not, the structure of our fingers is precisely the reason! The number of finger joints on each hand (excluding the thumb) makes it possible to count to 12 by using the thumb. Surprised at what a simple explanation that is? Well, things are going to get slightly more complicated…

Without artificial light, humans of this time period regarded sunlit and dark periods as two opposing realms rather than as part of the same day. Without the aid of sundials, dividing the dark interval between sunset and sunrise was more complex than dividing the sunlit period. It’s pretty interesting to learn how they managed to do this. Yes, nighttime division of time was based on the observation of stars! During the era when sundials were first used, however, Egyptian astronomers also first observed a set of 36 stars that divided the circle of the heavens into equal parts. The passage of night could be marked by the appearance of 18 of these stars, three of which were assigned to each of the two twilight periods when the stars were difficult to view. The period of total darkness was marked by the remaining 12 stars, again resulting in 12 divisions of night (another nod to the duodecimal system). Tables were produced to help people to determine time at night by observing the decans. Amazingly, such tables have been found inside the lids of coffins, presumably so that the dead could also tell the time. In the Egyptian system, the length of the day-time and night-time hours were unequal and varied with the seasons. In summer, day-time hours were longer than night-time hours while in winter the hour lengths were the other around. During the New Kingdom (1550 to 1070 B.C.), this measuring system was simplified to use a set of 24 stars, 12 of which marked the passage of the night. The clepsydra, or water clock, was also used to record time during the night, and was perhaps the most accurate timekeeping device of the ancient world. The timepiece--a specimen of which, found at the Temple of Ammon in Karnak, dated back to 1400 B.C.--was a vessel with slanted interior surfaces to allow for decreasing water pressure, inscribed with scales that marked the division of the night into 12 parts during various months.

Once both the light and dark hours were divided into 12 parts, the concept of a 24-hour day was in place. The concept of fixed-length hours, however, did not originate until the Hellenistic period, when Greek astronomers began using such a system for their theoretical calculations. Hipparchus, whose work primarily took place between 147 and 127 B.C., proposed dividing the day into 24 equinoctial hours, based on the 12 hours of daylight and 12 hours of darkness observed on equinox days. Despite this suggestion, laypeople continued to use seasonally varying hours for many centuries. (Hours of fixed length became commonplace only after mechanical clocks first appeared in Europe during the 14th century.)

Why is there 60 minutes in an hour?

Although it is no longer used for general computation, the sexagesimal system is still used to measure angles, geographic coordinates and time. In fact, both the circular face of a clock and the sphere of a globe owe their divisions to a 4,000-year-old numeric system of the Babylonians. The subdivision of hours and minutes into 60 comes from the ancient Babylonians who had a predilection for using numbers to the base 60. For example, III II (using slightly different strokes) meant three times 60 plus two or 182. We have retained from the Babylonians not only hours and minutes divided into 60, but also their division of a circle into 360 parts or degrees. What we have not retained is their division of a day into 360 parts called 'ush' that each equalled four of minutes in our time system.

The Greek astronomer Eratosthenes (who lived circa 276 to 194 B.C.) used a sexagesimal system to divide a circle into 60 parts in order to devise an early geographic system of latitude, with the horizontal lines running through well-known places on the earth at the time. A century later, Hipparchus normalized the lines of latitude, making them parallel and obedient to the earth's geometry. He also devised a system of longitude lines that encompassed 360 degrees and that ran north to south, from pole to pole. In his treatise Almagest (circa A.D. 150), Claudius Ptolemy explained and expanded on Hipparchus' work by subdividing each of the 360 degrees of latitude and longitude into smaller segments. Each degree was divided into 60 parts, each of which was again subdivided into 60 smaller parts. The first division, partes minutae primae, or first minute, became known simply as the "minute." The second segmentation, partes minutae secundae, or "second minute," became known as the second.

Minutes and seconds, however, were not used for everyday timekeeping until many centuries after the Almagest. Clock displays divided the hour into halves, thirds, quarters and sometimes even 12 parts, but never by 60. In fact, the hour was not commonly understood to be the duration of 60 minutes. It was not practical for the general public to consider minutes until the first mechanical clocks that displayed minutes appeared near the end of the 16th century. Even today, many clocks and wristwatches have a resolution of only one minute and do not display seconds.

Thanks to the ancient civilizations that defined and preserved the divisions of time, modern society still conceives of a day of 24 hours, an hour of 60 minutes and a minute of 60 seconds. Advances in the science of timekeeping, however, have changed how these units are defined. Seconds were once derived by dividing astronomical events into smaller parts, with the International System of Units (SI) at one time defining the second as a fraction of the mean solar day and later relating it to the tropical year. This changed in 1967, when the second was redefined as the duration of 9,192,631,770 energy transitions of the cesium atom. This recharacterization ushered in the era of atomic timekeeping and Coordinated Universal Time (UTC).

Interestingly, in order to keep atomic time in agreement with astronomical time, leap seconds occasionally must be added to UTC. Thus, not all minutes contain 60 seconds. A few rare minutes, occurring at a rate of about eight per decade, actually contain 61.

Why is there 7 days a week?

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.

Some of the earliest civilizations observed the cosmos and recorded the movements of planets, the Sun and Moon. The Babylonians, who lived in modern-day Iraq, were astute observers and interpreters of the heavens, and it is largely thanks to them that our weeks are seven days long.

The reason they adopted the number seven was that they observed seven celestial bodies — the Sun, the Moon, Mercury, Venus, Mars, Jupiter and Saturn. So, that number held particular significance to them.

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."

Other civilizations chose other numbers — like the Egyptians, whose week was 10 days long; or the Romans, whose week lasted eight.

The Babylonians divided their lunar months into seven-day weeks, with the final day of the week holding particular religious significance.

Why is there 28,29,30 or 31 days a month?

The ancient Romans, like ancient civilizations before them, based their concept of the month on the Moon. One Lunar cycle, that takes for the Moon to revolve around the Earth, is 29.5 days. A month is one such Moon cycle that was alternation of 29 and 30 days.

To understand the changes were made with months, we need to dive into the next question:

Why is there 365.25 days a year?

Detailed history

In Mesopotamia, where the Babylonians are the leading astronomers, the calendar is a simple lunar one. And a lunar calendar is still in use today in Islam. But such a calendar has one major disadvantage.

The length of a lunar month, from one new moon to the next, is 29.5 days. So twelve lunar months are 354 days, approximately 11 days short of a solar year. In a lunar year each of the twelve months slips steadily back through the seasons (as happens now with the Muslim calendar), returning to its original position only after 32 years.

In some lunar calendars an extra month is inserted from time to time to keep in step with the solar year. This happens in Mesopotamia and in republican Rome, and it remains the case today in the Jewish calendar.

Egyptian Calendar

In Egypt the temple priests derive much of their prestige from close attention to the stars, enabling them to give the impression of predicting natural events. The best example is their use of Sirius, the Dog Star. It rises above the horizon just before dawn at the time of year when the all-important flooding of the Nile is about to occur. Priests who can foretell this great event are powerful soothsayers.

This observation of Sirius also enables the Egyptians to become the first people to move from a lunar to a solar calendar.

But the Egyptian priests' observation of Sirius enables them to count the number of days in a solar year. They make it 365. They then very logically adjust the twelve months of the lunar year, making each of them 30 days long and adding 5 extra days at the end of the year. Compared to anybody else's calendar at the time this is very satisfactory. But there is a snag.

The priests cannot have failed to notice that every four years Sirius appears one day later. The reason is that the solar year is more exactly 365 days and 6 hours. The Egyptians make no adjustment for this, with the result that their calendar slides backwards through the seasons just like a lunar one but much more slowly. Instead of 32 years with the moon, it is 1460 years before Sirius rises again on the first day of the first month.

It is known from the records that in AD 139 Sirius rises on the first day of the first Egyptian month. This makes it certain that the Egyptian calendar is introduced one or two full cycles (1460 or 2920 years) earlier, either in 1321 or 2781 BC - with the earlier date considered more probable.

Julian Calendar

The Roman calendar introduced by Julius Caesar, and subsequently known as the Julian calendar, gets far closer to the solar year than any predecessor. By the 1st century BC reform in Rome has become an evident necessity. The existing calendar is a lunar one with extra months slipped in from to time in an attempt to adjust it. In Caesar's time this calendar is three months out in relation to the seasons.

On the advice of Sosigenes, a learned astronomer from Alexandria, Caesar adds ninety days to the year 46 BC and starts a new calendar on 1 January 45.

Sosigenes advises Caesar that the length of the solar year is 365 days and six hours. The natural solution is to add a day every fourth year - introducing the concept of the leap year. The extra day is added to February, the shortest of the Roman months.

Spread through the Roman empire, and later throughout Christendom, this calendar proves very effective for many centuries. Only much later does a flaw yet again appear. The reason is that the solar year is not 365 days and 6 hours but 365 days, 5 hours, 48 minutes and 46 seconds. The difference amounts to only one day in 130 years. But over the span of history even that begins to show. Another adjustment will eventually be necessary.

While Julius Caesar is improving on the solar calendar of 365 days, a similar calendar has been independently arrived at on the other side of the Atlantic. Devised originally by the Olmecs of central America, it is perfected in about the 1st century AD by the Maya.

The Maya, establishing that there are 365 days in the year, divide them into 18 months of 20 days. Like the Egyptians (who have 12 months of 30 days), they complete the year by adding 5 extra days at the end - days which are considered to be extremely unlucky for any undertaking. An unusual aspect of the Mayan system is the Calendar Round, a 52-year cycle in which no two days have the same name.

Gregorian calendar: 1582-1917

By the 16th century the seemingly minor error in the Julian calendar (estimating the solar year to be 11 minutes and 14 seconds shorter than it actually is) has accumulated to a ten-day discrepancy between the calendar and reality. It is most noticeable on occasions such as the equinox, now occurring ten days earlier than the correct calendar dates of March 21 and September 23.

Pope Gregory XIII employs a German Jesuit and astronomer, Christopher Clavius, to find a solution. Calculating that the error amounts to three days in 400 years, Clavius suggests an ingenious adjustment.

His proposal, which becomes the basis of the calendar known after the commissioning pope as Gregorian, is that century years (or those ending in '00') should only be leap years if divisible by 400. This eliminates three leap years in every four centuries and neatly solves the problem. The result, in the centuries since the reform, is that 1600 and 2000 are normal leap years, but the intervening 1700, 1800 and 1900 do not include February 29.

Gregory puts the proposal into immediate effect in the papal states, announcing that the day after October 4 in 1582 will be October 15 - thus saving the lost ten days.

The pope's lead is followed in the same year by Spain, Portugal, France and most Italian states. The German-speaking Roman Catholic states comply in 1583.

Other Christian realms drag their feet on the issue, reluctant to admit that the pope in Rome has a point. The Lutheran states of Germany change in 1700. Great Britain delays until 1752, by which time the gap is eleven days. Some of the British prove exceptionally dim over the issue, fearing that their lives are being shortened and in places even rioting for the return of the missing days. Imperial Russia never makes the change; it is introduced after the revolution, in 1918. (Potentially confusing dates, near the change-over years, are identified by historians with the codes OS or Old Style for the Julian version and NS or New Style for the Gregorian equivalent.)

More precise measurements in the 20th century have introduced a further refinement of the Gregorian calendar, though not one of immediate significance. As adjusted for pope Gregory, the present system adds one day in every 3,323 years. The accepted solution is that years divisible by 4000 will not be leap years.

February 29 will therefore be dropped unexpectedly in 2000 years' time. In4000, even though the year is divisible by 400, March 1 will follow February 28 in the normal way. Julius Caesar and Sosigenes would no doubt be impressed by this ultimate refinement of their system, making it accurate to within one day in 20,000 years.

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 of saying it. In fact, EarthSky reports that Jewish academics have used B.C.E./C.E. for over 100 years.

Summary

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.

Adjustments of moon calendar to sun calendar:

Roman Calendar

The ancient Romans started using a 10-month calendar in 738 B.C. that included the following months: Martius, Aprilis, Maius, Junius, Quintilis, Sextilis, September, October, November, and December. The names Quintilis through December derived from the Latin words for five through ten.

To account for the remaining 60 or so days, Januarius was added to the beginning of the year and Februarius to the end of the year during Numa's reign around 700 B.C. The calendar stayed in that order until 452 B.C. when a small council of Romans, called the Decemvirs, moved February to follow January. However, this resulted in a year of only 354 days while the orbital period of the Earth is 365.2422 days. As a result, the calender became out of sync with seasons which was bad. This was initially corrected in an arbitrary way by adding a 13th month, but soon the calender was thrown into severe confusion.

Julian Calendar

In 46 B.C., Julius Caesar reformed the calender by ordering the year to be 365 days in length and to contain 12 months each month have either 30 or 31 days with the exception of Februarius, which had 29 days. To account for the extra 0.2422 days, every fourth year was made a leap year, which adding to Februarius an extra day and bringing it to 30 days. This made the average length of a year to be 365.25 days. Quintilis was later renamed Julius in his honor.

Likewise, Sextilis later became Augustus to honor Augustus Caesar. Augustus was also given an extra day (taken away from Februarius), so that Augustus and Julius would have an equal number of days (31) and in Februarius became 28 days.

Gregorian Calendar

In addition, a modification was made that century years that were not divisible by 400 would not be considered as leap years. For example, 2000 would be a leap year while 2100 would not. This made the year sufficiently close to the actual year and this calender is called the Gregorian calender.

As the year is now set up to follow the seasons accurately, it no longer follows the phases of the Moon.

Year Zero

A year zero does not exist in the Anno Domini (AD) calendar year system commonly used to number years in the Gregorian calendar (nor in its predecessor, the Julian calendar); in this system, the year 1 BC is followed directly by year AD 1. However, there is a year zero in both the astronomical year numbering system (where it coincides with the Julian year 1 BC), and the ISO 8601:2004 system, the interchange standard for all calendar numbering systems (where year zero coincides with the Gregorian year 1 BC)
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Leap Second

If it's not enough rules of leap year that dependable either or not it is divisible by 400, here comes Leap Second.

A leap second is a one-second adjustment that is occasionally applied to Coordinated Universal Time (UTC), to accommodate the difference between precise time (International Atomic Time (TAI), as measured by atomic clocks) and imprecise observed solar time (UT1), which varies due to irregularities and long-term slowdown in the Earth's rotation. The UTC time standard, widely used for international timekeeping and as the reference for civil time in most countries, uses TAI and consequently would run ahead of observed solar time unless it is reset to UT1 as needed. The leap second facility exists to provide this adjustment.

Because the Earth's rotation speed varies in response to climatic and geological events,[1] UTC leap seconds are irregularly spaced and unpredictable. Insertion of each UTC leap second is usually decided about six months in advance by the International Earth Rotation and Reference Systems Service (IERS), to ensure that the difference between the UTC and UT1 readings will never exceed 0.9 seconds.
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Final Thoughts

If a month was build on a moon phase but not represent it anymore...
If a count of years were from Jesus birth but now not represent it either...
If a week was build on number of planet that ancient people could count...
If a time clock was build in sexagesimal system that not really in use now...

What are really the benefits of the existing system? Why we still use it?

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