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The Gregorian Calendar

The Gregorian calendar is today's internationally accepted civil calendar and is also known as the Western or Christian calendar.

Tourists visiting the Papal Basilica of St. Peter in the Vatican City Rome, Italy - November 14 2015

The Vatican, 'birth place' of the Gregorian calendar.

Rome's Vatican City is the 'birth place' of the Gregorian calendar, named after Pope Gregory XIII, who introduced it.

©iStockphoto.com/Antonio Gravante

12 Irregular Months

The Gregorian Calendar is the most widely used calendar in the world today. It is a solar calendar based on a 365-day common year divided into 12 months of irregular lengths.

11 of the months have either 30 or 31 days, while the second month, February, has only 28 days during the common year.

However, nearly every four years is a leap year, when one extra – or intercalary – day, is added on 29 February, making the leap year in the Gregorian calendar 366 days long.

Replaced Julian Calendar

The Gregorian calendar's predecessor, the Julian Calendar, was replaced because it was too inaccurate. It did not properly reflect the actual time it takes the Earth to circle once around the Sun, known as a tropical year.

Realigned With the Sun

The Julian calendar's formula to calculate leap years produced a leap year every four years. This is too often, and eventually the Julian calendar was several days out of sync with the fixed dates for astronomical events like equinoxes and solstices.

The introduction of the Gregorian calendar allowed for the realignment with events like the vernal equinox and winter solstice.

New Leap Year Formula

The Gregorian calendar was first adopted in Italy, Poland, Portugal and Spain in 1582, and included the following changes:

  • New formula for calculating leap years:
    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
  • 10 days were dropped in October 1582
  • New rules for calculating Easter-dates

Is Any Calendar Perfect?

The more advanced leap year formula makes the Gregorian calendar far more accurate than the Julian. However, it is not perfect either. Compared to the tropical year, it is off by one day every 3236 years.

Who Designed the 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.

Topics: Calendar, Dates, Months, History, Seasons, Solstice, Equinox, Leap Year, Weekdays

The Gregorian Calendar
Used inMost of the world
Calendar typeSolar
Accuracy1 day in 3236 years
Number of daysCommon year: 365
Leap year: 366
Number of months12
Correction mechanismLeap day


Calendar Types

  1. Gregorian Calendar
  2. Julian Calendar
  3. Revised Julian Calendar
  4. Islamic Calendar
  5. Jewish Calendar
  6. Persian Calendar
  7. Mayan Calendar
  8. Chinese Calendar
  9. Roman Calendar
  10. Ethiopian Calendar
  11. Switch from Julian to Gregorian
  12. Is There a Perfect Calendar?
  13. What Do CE and BCE Mean?

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