Calendars Mayan, Roman, Julian and Gregorian.

From Time and Date:

Gregorian Calendar

The Gregorian calendar is today’s internationally accepted civil calendar and is also known as the “Western calendar” or “Christian calendar”. It was named after the man who first introduced it in February 1582: Pope Gregory XIII.

Calendar features

The calendar is strictly a solar calendar based on a 365-day common year divided into 12 months of irregular lengths. Each month consists of either 30 or 31 days with 1 month consisting of 28 days during the common year. A Leap Year usually occurs every 4 years which adds an extra day to make the second month of February 29 days long rather than 28 days.

Realigned with the equinox

The Gregorian calendar reformed the Julian calendarbecause the Julian calendar introduced an error of 1 day every 128 years. The introduction of the Gregorian calendar allowed for the realignment with the equinox, however a number of days had to be dropped when the change was made.

The Gregorian calendar was first adopted in Italy, Poland, Portugal and Spain in 1582. The Gregorian reform consisted of the following changes:

10 days were dropped in October 1582.
New rules were set to determine the date of Easter.
The rule for calculating Leap Years was changed to include that a year is a Leap Year if:
1. The year is evenly divisible by 4;
2. If the year can be evenly divided by 100, it is NOT a leap year, unless;
3. The year is also evenly divisible by 400. Then it is a leap year.

Is there a perfect calendar?
February 30 was a real date

For example, the years 1900, 2100, and 2200 are not Leap Years. However, the years 1600, 2000, and 2400 are Leap Years.

The Julian calendar is currently (between the years 1901 and 2099) 13 days behind the Gregorian calendar because too many Leap Years were added. The Gregorian calendar is off by about 1 day every 3236 years.

Who designed the Gregorian calendar?

Although the Gregorian calendar is named after Pope Gregory XIII, it is an adaptation of a calendar designed by Italian doctor, astronomer and philosopher Luigi Lilio (also known as Aloysius Lilius). He was born around 1510 and died in 1576, six years before his calendar was officially introduced.

Other calendars

More information

Calendar tools

Calendar for 2013
Calendar Generator – Create a calendar for any year
Duration between two dates – Calculates number of days

The Gregorian calendar:

From Conlatio (at wordpress)

Also called the Western calendar and the Christian calendar, is internationally the most widely accepted civil calendar.^[1]^[2]^[3] It was introduced by Pope Gregory XIII, after whom the calendar was named, by a decree signed on 24 February 1582; the decree, apapal bull, is known by its opening words, Inter gravissimas.^[4] The Gregorian calendar was adopted initially by the Catholic countries of Europe, with other countries adopting it over the following centuries.

The motivation for the Gregorian reform was that the Julian calendar assumes that the time between vernal equinoxes is 365.25 days, when in fact it is presently almost 11 minutes shorter. The discrepancy results in a drift of about three days every 400 years. At the time of Gregory’s reform there had already been a drift of 10 days since Roman times, resulting in the spring equinox falling on 11 March instead of the ecclesiastically fixed date of 21 March, and moving steadily earlier in the Julian calendar. Because the spring equinox was tied to the celebration of Easter, the Roman Catholic Church considered this steady movement in the date of the equinox undesirable. continue Reading >>

The Curious History of the Gregorian Calendar

Eleven days that never were

by Ben Snowden

September 2, 1752, was a great day in the history of sleep.

That Wednesday evening, millions of British subjects in England and the colonies went peacefully to sleep and did not wake up until twelve days later. Behind this feat of narcoleptic prowess was not some revolutionary hypnotic technique or miraculous pharmaceutical discovered in the West Indies. It was, rather, the British Calendar Act of 1751, which declared the day after Wednesday the second to be Thursday the fourteenth.

Prior to that cataleptic September evening, the official British calendar differed from that of continental Europe by eleven days—that is, September 2 in London was September 13 in Paris, Lisbon, and Berlin. The discrepancy had sprung from Britain’s continued use of the Julian calendar, which had been the official calendar of Europe since its invention by Julius Caesar (after whom it was named) in 45 B.C.

Caesar’s calendar, which consisted of eleven months of 30 or 31 days and a 28-day February (extended to 29 days every fourth year), was actually quite accurate: it erred from the real solar calendar by only 11½ minutes a year. After centuries, though, even a small inaccuracy like this adds up. By the sixteenth century, it had put the Julian calendar behind the solar one by 10 days.

In 1582, Pope Gregory XIII ordered the advancement of the calendar by 10 days and introduced a new corrective device to curb further error: century years such as 1700 or 1800 would no longer be counted as leap years, unless they were (like 1600 or 2000) divisible by 400.

Read more by Ben Snowden: The Gregorian Calendar History