Time and calendar

The (Catholic) Easter date was fixed by the Council of Nicea in 325 AD. The rule is as follows: Easter is the first Sunday after the first full moon of spring. Taking into account modern astronomical calculations, in 2019,
  • spring begins on March 20 at 22:58 (universal time);
  • the first full moon that follows takes place on March 21 at 2:43 (universal time).
Therefore, Easter would fall on the first Sunday following, i.e. on March 24, 2019. However, the Easter date in 2019 is April 21. The reason why the two dates do not coincide is that the authorities of the Catholic Church do not want to depend on astronomers. For this reason, they have introduced the following simplifications into their calculations: 1) Early spring (in the northern hemisphere): In astronomy, it coincides with the vernal equinox, the moment when the terrestrial equatorial plane crosses the centre of the Sun and the Earth's North Pole emerges from its winter night. The equinox date fluctuates in the calendar, in part because the Earth's period of revolution around the Sun is 365.2422 days instead of 365-366 calendar days (other factors influence the calculation of the spring date, but their influence is too small to be mentioned here). As a result, early spring is between March 19 and 21. The church has decided to set the date from spring to March 21. 2) The dates of the full moon: It is not easy to determine the exact orbit of the Moon and the corresponding lunar phases. In the 5th century BC, the Greek Meton of Athens found an elegant solution to simplify this problem. After a number of observations he concluded that the Sun and Moon are in the same relative position with respect to the Earth after 235 lunar months, or 19 years. This cycle has become the basis for the calculation of the full moon in the Catholic Church. In 1582, this rule was adapted to the new Gregorian calendar, our current calendar. The rule for calculating the Easter date therefore remains valid, but we must also take into account the two simplifications mentioned above. In other words,
  • spring begins on March 21;
  • the first full moon after March 21, 2019 (according to Meton calculations) takes place on April 20, 2019.
Therefore, Easter falls on the Sunday following this full moon, i.e. on April 21, 2019. In addition to 2019, there are other years when the "ecclesiastical" Easter date does not correspond to the "astronomical" Easter date. In the thousand years following 1582, the year of the Gregorian calendar reform, there were 84 such occurrences. The previous date was in 1981, the next will be in 2038. The German mathematician Carl Friedrich Gauss published in 1800 a mathematical algorithm to calculate the Easter date for a given year. He uses different terms of the comput such as the golden number or the epacte. This algorithm is still used today, sometimes in a simplified version. Links: Easter dates from 1583 to 3000 Algorithm for the computation of the Easter date
Since 2018, we no longer give information on religious calendars. We now only give information on the Belgian civil calendar, which is based on the Gregorian calendar.
The length of the year in the Western calendar has its origin in the Roman calendar. Since 46 BC, Julius Caesar wanted to establish 365-day years with a leap year every 4 years. However, it will take some time before the Julian calendar is properly applied. The scope of this calendar refers to the length of the tropical year. This is the time it takes for the Sun to return to the same position in the sky. During this time, the Sun moves from its northernmost position to its southernmost position relative to the equator and vice versa. This results in the frequency of the seasons that the calendar wants to reflect. A tropical year lasts 365.242190 days or 365 days 5 hours 48 minutes 45.2 seconds, so it is not an integer number of days. A good approximation was made by introducing a year of 366 days every 4 years as already provided for in the Julian calendar. This gives a year with an average duration of 365.25 days. Over time, a small difference (0.0078 days per year) began to be a problem. Thus, because of the imperfection of the Julian calendar, spring in the 16th century was moved to March 11. In order to put everything in order, Pope Gregory XIII decided in 1582 that October 4 should be followed by October 15. In addition, at the request of the astronomers of the time, he implemented the following new rule: a year divisible by 100 will not be leapfrogging, unless it is a multiple of 400. the first rule alone produces an average year of 365.24 days every 100 years. The second rule is to get very close to the tropical year, i.e. 365.2425 days every 400 years. It is only after 3000 years that the inaccuracy deviates by one day. However, this timetable was not immediately adopted by all countries. Russia waited until 1917, which is why the October revolution is now commemorated in November. Sweden became so confused in the 18th century that it was necessary to give 30 days in February in 1712. Greece adopted the Gregorian calendar in 1923. Several people noted that some sources contradict each other. Thus, the southern regions of present-day Belgium moved to the Gregorian calendar from 1582 to 1583. In some places (Flanders and Hainaut) Christmas Day was not celebrated in 1582, since December 21, 1582 was immediately followed by January 1, 1583. Even if Brabant and Zeeland had made the transition earlier (14-25 December), it was not without difficulties. The Liege échevinage introduced the Gregorian reform in February 1583. You can find more information about calendars on, for example, The Calender FAQ.
A full century is 100 years old. 100 years must therefore pass before a new century can begin. Since our era begins with the year 1 AD, the 1st century ended with the end of the year 100. Thus, the year 1900 had to end first before the 19th century could be finished. The 20th century also lasted 100 years and ended with the beginning of 2001. The reasoning is the same for a millennium. A millennium lasts 1000 years. The second millennium began on 1 January 1001 and ended on 31 December 2000. Each era begins with year 1. There is no such thing as year 0.

A brief historical overview

Among the Romans, who laid the foundations of our calendar, the era began with the foundation of Rome, fixed in 753 BC (Ab Urbe condita or AUC). However, there were other starting dates. Since the 3rd century AD, the years have been calculated from the ascent of the Emperor Diocletian. Since he persecuted Christians, this time was called the era of the martyrs. In the 6th century, the monk Dionysius Exiguus (Dionysius the Little) began to count from the year that, according to him, was the year of Christ's birth. Thus, 248 Anno Diocletiani was in fact the year 532 AD. According to Dionysius, Jesus Christ was born on December 25 of the year before year 1, he did not know the number zero, but there is no era in which a year 0 would be placed. In the 8th century, the English Father Bede the Venerable also began to count the years before Christ. The year 1 BC was therefore just before the year 1 AD. The year 1 BC was therefore the last year of the 1st century BC. It should be noted that this Christian way of counting years was used by the Church in the Middle Ages and much later in the civil world. There were many other ways to count. There were controversies about the date of the beginning and in particular about the year of Christ's birth and Herod's death in 4 BC, which should be brought forward by a few years, to around 6 or 7 BC. You can find more information about calendars on The Calender FAQ web page.