The origin of…Calendar

“Don’t be fooled by the calendar. There are only as many days in the year as you make use of.” Charles Richards

Post 49 Calendar

The simplest calendar system counts time periods from a reference date, like the Julian day, named for Julius Scaliger, who invented the concept. The Julian day is the continuous count of days since the beginning of the Julian Period, used by astronomers. The Julian Day Number is the number belonging to a day in the Julian day count starting from noon Greenwich Mean Time, with noon January 1, 4713 BC in the Julian calendar (November 24, 4714 in the Gregorian calendar) considered to be the number 0. So, the JDN for October 2, 2014 is 2,456,933 (if you want to calculate it yourself, check

The Julian day is not to be confused with the Julian calendar, the calendar introduced by Julius Caesar in 46 BC, which was a reform of the Roman calendar. The Roman calendar was a lunar calendar, possibly based on the Greek lunar calendars. The original Roman calendar was the Calendar of Romulus, the founder of Rome, who introduced it in the 700´s BC. This calendar had 10 months: Martius (31 days), Aprilis (30 days), Maius (31 days), Iunius (30 days), Quintilis (31 days), Sextilis (30 days), September (30 days), October (31 days), November (30 days) and December (30 days). This means that the calendar had 304 days, and the remaining 61 days that fell in winter seem to have been ignored by the Romans.
Around 713 BC Numa Pompilius improved the calendar and added two months, January (29 days) and February (38 days). Since the Romans thought odd numbers were lucky, all the 30 day-months were transformed into months with 29 days. These changes made the Roman calendar 355 days long and to make the calendar correspond with a solar year, every other year a leap month called Mensis Intercalaris, or Mercedonius (22 or 23 days), was inserted in February.
Another refinement took place in 46 BC, when Julius Caesar changed the calendar. The Julian calendar has a regular year of 365 days divided into 12 months and leap day is added to February every four years. Although Greek astronomers already knew that a solar year was a few minutes shorter than 365.25 days, the Julian calendar did not compensate for this. So, the calendar gained approximately three days every four hundred years compared to observed equinox times. This discrepancy was corrected by the Gregorian calendar in 1582. It is named after Pope Gregory XIII who refined the Julian calendar to change the date for the celebration of Easter to the time of the year that the First Council of Nicaea had decided upon in 325. The changes made consisted of reducing the number of leap years from 100 to 97 in four hundred years and a new calculation of the lunar cycle to calculate the annual date of Easter. The change in leap years meant that the mean length of a calendar year changed from 365.25 to 365.2425 days.

The new calendar was adopted in 1582 in four Catholic countries: the Polish-Lithuanian Commonwealth, Portugal, Spain and most of Italy, Many Protestant countries initially objected to adopting a Catholic innovation and many didn´t change to the new calendar until 1700. Countries in Asia started using the Gregorian calendar as of the 19th century (Japan 1872, Korea 1895, China 1912). Now most countries use the Gregorian calendar, although there are some exceptions like Saudi Arabia (where the Islamic calendar, but based on a calculated astronomical moon, is used), Ethiopia, Nepal, Iran, and Afghanistan. Some countries use other calendars together with the Gregorian calendar like India, Pakistan, Bangladesh, Burma, Indonesia, Malaysia and Israel. In some of these countries the Islamic calendar is used for religious purposes and the Gregorian for civil purposes.

The Islamic calendar starts in 622 AD with the emigration of the Islamic prophet Mohammed from Mecca to Medina. It is a lunar calendar and has 12 months and 354 days. The first day of each month is the day of the first observation of the crescent moon (hilal), right after sunset. If the hilal is not seen right after the 29th day of a month (due to weather, optical properties of the atmosphere and the location of the observer), then the day that begins at that sunset is the 30th. Each Islamic country has its own monthly observation of the new moon, which leads to different beginnings of each month. So, it is difficult to give accurate information in advance about when a new month will start. In some countries, like Malaysia and Indonesia, a new month starts at sunset on the first day that the moon sets after the sun.


  1. a document, chart, etc., that shows the days, weeks, and months of a year
  2. a list or schedule of events or activities that occur at different times throughout the year
  3. a particular system for organizing the days of the year by month (source: Merriam Webster).

The English calendar, Dutch kalender, Spanish calendario and Italian calendario come from Latin calendarium, account book, from calendae/kalendae, calends, the first day of the Roman month, when debts fell due and accounts were totaled, from calare, to announce solemnly/call out, as the priests did in proclaiming the new moon that marked the calends, from the Proto Indo European root kele-, to call, shout (source:

Calendar in other languages:
Dutch: kalender
Spanish: el calendario
Italian: il calendario


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