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The International Calendar

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The international calendar currently followed is Julian calendar with the Gregorian correction or its simply called the Gregorian Calendar.

This is a solar calendar i.e year is based on the time taken for the earth to revolve round the sun. It consists of 12 months in an year. Each month consists of a specified number of days. Only the second month February consist 28 days in common years and 29 days in leap years. Thus common years have 365 days a year and leap years have 366 days a year.

What is a leap year?

Its an year when there is an extra-day (i.e February 29th). This intercalation of day is to adjust the discrepancy arising out of the normal year period of 365 days and the actual earth's revolution time of earth 365.242199 (365 days, 5 hours, 48 minutes, 46 seconds).

How is the leap year determined?

As per Julian calendar every year divisible by four was a leap year. This led to a discrepancey of 3.12 extra days over 4 centuries. This was corrected by Pope Gregory. As per this the century years are leap years only if its divisible 400. There is also further refinement suggested that years divisible by 4000 are non-leap or common years. Even this refinement it will still lead to discrepancy of a day over 20000 years.

As per the current Gregorian calendar the determination of the leap year is as follows :

  1. All non-century years divisible by four are leap years.
  2. All century years divisible by 400 is a leap year.
    Which means 1900 & 2100 are not a leap years while year 2000 is a leap year.

As per the refinements to Gregorian calendar, we have this additional clause :

  1. All years divisible by four thosand (4000) are common years or non-leap years.

Earth's revolution, & calculations related to leap years & correction

The year is based on the time taken for one revolution of the Earth (Around the Sun.) The tropical year (the interval between successive passages of the Sun through the vernal equinox.) consists of 365.242199 mean solar days, i.e., 365 days 5 hours 48 minutes 46 seconds. (The mean solar day is the average interval between two passages of the Sun across the meridian.)

As per Julian calendar, there is leap year every 4 years.

  So the descrepancy for every four years is = 365*3+366 - 365.242199*4
                                             = 0.0312037 days extra for 4 yrs
  => every 400 years the discrepancy is      = 0.0312037 *100 days
                                             = 3.12037 days extra for 400 yrs
With the Gregorian correction, century years are leap years only if divisible by 400
  So Gregorian calendar reduces 3 days for every 400 years.
  With this the net discrepancy is           = 3.12037 -3
                                             = 0.12037 days extra for 400 yrs
  
  => every 4000 yrs the discrepancy is       = 0.12037*10
                                             = 1.2037 days extra for 4000 yrs
A further refinement is, every year divisible by 4000 will be a common or non-leap year.
  So this refinement reduces 1 day every 4000 years.
  With this the net discrepancy is           = 1.2037 - 1
                                             = 0.2037 days extra for every 4000 yrs
  
  => Over the period of 20000 yrs there will be discrepancy of about a day

As shown above the current calendar even with refinements to Gregorian correction still needs adjustments.

Weeks, Working days, Holidays

In most of the calendars, 7 days form a week. Many of the countries follow 5 days a week as working days. Some of the countries have 6 days as working days.

In most of the non-Islamic countries following Sunday is non-working day. Also, Saturday is non-working if the country follows 5 day week.

In Islamic countries, however, Friday is a holiday. Those Islamic countries following 5 working-days a week have Thursday as non-working. But for business transactions between Islamic & non-Islamic countries there will be 4 days which are non-working in either of the countries, So some Islamic countries like Pakistan have Fridays & Saturdays instead.

Apart from these, each country has a fixed set of days which are declared as holidays. These are different in each country and depends onn the national events, religious festivities of the country.

Origins of today's International Calendar.

Julius Caesar with the help of Alexandrian astronomer Sosigenes, reformed the Roman calendar into solar. He took the length of the solar year as 365.25 days. The year was divided into 12 months, all of which had either 30 or 31 days except February, which contained 28 days in common (365-day) years and 29 in every fourth year . Due to misunderstandings, the calendar was not established in smooth operation until AD 8. This calendar is known as Julian Calendar.

Sosigenes had overestimated the length of the year as shown in calculations above, and by the mid-1500s, the cumulative effect of this error had shifted the dates of the seasons by about 10 days from Caesar's time. In 1582, Pope Gregory XIII issued a papal bull drawn up by Jesuit astronomer Christopher Clavius , to restore the calendar to the seasonal dates of AD 325 with an adjustment of 10 days (October 4th, 1582 is to be followed by October 15th,1582) and a clause that centennial years are leap year only if divisible by 400. This came to be known as Gregorian correction to Julian calendar or simply known as Greogorian calendar.

The Gregorian correction was not immediately accepted everywhere. Most of the Roman Catholic states (Italian states, Portugal, Spain, and the German Catholic states accepted within an year) adopted the improved dating system by 1587 . Some Protestant states (the Protestant German states in 1699) embraced it around the beginning of the 18th century, but a number of others, such as Great Britain and its colonies, did in 1752. Sweden in 1753. Japan (in 1873), China (in 1912), the Soviet Union (in 1918), Greece (in 1923), to name only a few, adopted the Gregorian rules much later. A few dating systems besides the Gregorian calendar still remain in use. The Muslim calendar, for example, has been retained by most Arab countries. The traditional Hindu and Jewish calendars continue to be used for religious purposes.


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