Gregorian Calendar

What is the Gregorian calendar?

The Gregorian calendar is the time division system currently used in the West. It was promoted by Pope Gregory XIII and implemented in 1582 in Europe.

This calendar was created to correct the inaccuracy of the Julian calendar, which was created in 46 BC. C by Emperor Julius Caesar and remained in force until 1582.

The Gregorian calendar is made up of 12 months of 31, 30, or 28 days, resulting in a 365-day year. Every four years an additional day is added to the month of February to have a year of 366 days, with some exceptions. It is when we say that it is a leap year.

The exceptions to determining leap years in the Gregorian calendar are: years divisible by 100 are never leap, and years divisible by 400 are always leap years.

Gregorian calendar composition


In the Gregorian calendar the years are counted taking as reference the year of birth of Jesus of Nazareth. For that reason, time is divided into two great historical periods: before Christ (BC) and after Christ (AD). The year of Jesus’ birth would be the year 1 AD, while the year before his birth is considered the year 1 BC.

In turn, a year can be:

  • Common : is the year that has 365 days.
  • Leap : are the years that have 366 days. This occurs every four years, with some exceptions, and the additional day is added to the month of February, which that year goes from having 28 days to having 29.
  • Secular – These are the years ending in 00 (such as 1900 or 2000). They are the years that determine the end of a century and are never leap.


A month is a division of time in the calendar, made up of a period of 28, 30 or 31 days. February is the only month with 28 days (except for leap years, in which it has 29). The rest of the months alternate the period of 30 or 31 days as follows:

30-day months : April, June, September and November.
31-day months : January, March, May, July, August, October and December.


A week is a period of seven consecutive days. A month has approximately four weeks and a year has 52 weeks.

In the Gregorian calendar, the counting of the weeks begins in January with week one, and ends in December with week 52.

For their part, most of the names of the days of the week in Spanish are a derivation of the Latin names of some celestial bodies. This does not apply to Saturday and Sunday, which has another origin.

Monday (from Latin Moon)

Tuesday (from Latin Mars which means Mars)

Wednesday (from Latin Mercurius, which means Mercury)

Thursday (from Latin Iuppiter which means Jupiter)

Friday (from Latin Venus)

Saturday (from the Hebrew shabbat, which means day of rest)

Domingo (from Latin dominica, which means Lord’s day)

According to the ISO 8601 standard, dedicated to the regulation of time data, the week starts on Monday and ends on Sunday.


The day is the basic unit of time in the Gregorian calendar. One day is made up of:

  • Hours : a day has 24 hours. Every hour is a 60 minute unit of time.
  • Minutes : in a day there are 1440 minutes. Each minute has 60 seconds.
  • Seconds : A day has 86,400 seconds.

Leap years in the Gregorian calendar

The Gregorian system has a couple of exceptions to determine which years are leap and which are not. The exceptions are:

  • Years that are divisible by 400 (such as the year 2000) are leap years.
  • Years that are divisible by 100 (like the year 1900, for example) are not leap years.

These exceptions gave the Gregorian system an even greater advantage over the Julian system that was used until 1582. The Julian calendar had a calculation error that caused it to accumulate one day every 128 years, generating an alteration in the counting of time. On the other hand, with these modifications in the Gregorian system, the lag is one day every 3300 years.

History of the Gregorian calendar

In 325 the Council of Nicaea was held in the Roman Empire. It was a high-ranking meeting in which the Christian bishops determined, among other issues, the date of celebration of Easter or Resurrection Sunday. This date is very important because it determines the rest of the celebrations of the liturgical calendar of Christianity.

To do this, the date of the spring equinox had to be determined first. That year, the equinox was dated March 21, but the inaccuracy of the Julian calendar accumulated one day every 128 years, which generated a lag that in 1582 already reached 10 days.

For this reason, in 1582 the equinox was dated March 11, a date very distant from that originally established at the Council of Nicea.

Calendar modification process

To correct this gap, in 1580 a Calendar Commission had already been established in Europe, made up of the Italian astronomer Luis Lilio and scholars from the University of Salamanca.

The correction of the calendar resulted in that March 21 could be maintained as a stable date of the spring equinox, without the need for future alterations.

Promulgation and implementation of the Gregorian calendar

In October 1582 Pope Gregory XIII promulgated the modification of the calendar in a papal bull called Inter Gravissimas. The new system was first implemented by Christian countries and colonies and by the middle of the 20th century had been adopted in almost the entire Western world.

However, some cultures retained their own systems of measuring time. Some examples are the Hebrew calendar, the Buddhist calendar, the Islamic calendar, or the Chinese calendar.

Although what was done was to modify the Julian calendar, little by little it became known as the Gregorian calendar in honor of its main popularizer, Pope Gregory XIII.

Gregorian calendar and Julian calendar

The Gregorian calendar is just a correction to the Julian calendar, which was based on a 365-day solar year. The inaccuracy of the Julian calendar comes from calculating a solar year (the time it takes for the Sun to make one complete revolution on its ecliptic) in 365, 25 days.

These tenths (0.25) correspond to 11 minutes and 14 extra seconds, which accumulated until completing 24 hours every 128 years, generating an alteration in the calendar.

The contribution of the Gregorian calendar was to establish that the exact duration of a year is 365 days, 5 hours, 48 ​​minutes and 45.16 seconds. This made it a much more accurate system than the previous one and for that reason it was promulgated as the new system of measuring time, first in Europe and then in the rest of the West.