From Julian to Gregorian: The Progress of Timekeeping

The Gregorian calendar is the world’s most widely-used civil calendar. Named for Pope Gregory XIII, who introduced it in 1582, it replaced the Julian calendar, which had been demarcating European days since Julius Caesar implemented it in 46 B.C.

Although intended to correct irregularities that the Roman emperor’s system had created, the Gregorian calendar was so controversial that some countries, such as Russia, Greece, and Turkey, did not adopt it until the early twentieth century.


The main problem with the Julian calendar was that it did not accurately reflect the length of the solar year, or the actual time it takes for the earth to go around the sun. A miscalculation of eleven minutes caused the calendar to gradually lose sync with the seasons. This concerned Pope Gregory, because the Easter holiday was supposed to coincide with the spring equinox, and the Julian system was pushing the two events further apart with each passing year.

Becoming the Official Calendar for the Catholic Church

The Italian scientist Aloysius Lilius (also known as Luigi Lilio) had already studied the matter and proposed a system that, in addition to other adjustments, added leap days in years divisible by four, unless the year was also divisible by 100. His suggestions were presented to Gregory, who approved them and passed them on to the calendar reform commission in 1575.

In 1577, the commission published a document titled, “Compendium novae rationis restituendi kalendarium” (“Compendium of a New Plan for the Restitution of the Calendar”), and on October 15, 1582, the Gregorian calendar became the official calendar for the Catholic Church.

The new calendar introduced the following reforms:

  • New rules were adopted for determining the date of Easter;
  • Ten days were dropped from the existing calendar. It was decreed that the day following October 4, 1582 would be October 15, 1582;
  • The rule for determining leap years was altered. Under the Julian calendar a year became a leap year if it was divisible by four. The Gregorian system stated that a year was a leap year only if it was divisible by four but not 100, or if it was divisible by 400;
  • The extra day in a leap year was changed from the day before February 25th to the day following February 28th.

Although the Pope’s calendar reforms had no power beyond the Church, Catholic countries, such as France and Spain, quickly adopted it for their civil matters. Protestants in Europe initially resisted the changeover, wanting nothing to do with a papal creation, but in time they gradually capitulated. The staunchly Protestant Germany switched over in 1700, followed by England in 1752.

“Give Us Back Our 11 Days”

When the British Parliament implemented the Gregorian calendar to align Great Britain and its overseas colonies with the rest of Western Europe, eleven days disappeared literally overnight. People went to bed on the night of September 2 and woke up on the morning of September 14. Some accounts allege that rioters took to the streets, demanding that the British government “give us (back) our 11 days.” In the American colonies across the Atlantic, Benjamin Franklin embraced the change, writing in his diary, “It is pleasant for an old man to be able to go to bed on September 2, and not have to get up until September 14.”

Sir John Herschel proposed a modification during the early nineteenth century: 969 leap days every 4000 years, which would reduce the average year to 365.24225 days instead of the 365.2425 under the Gregorian system. The year 4000 and its multiples would be common years instead of leap ones. The change was never adopted, although it continues to be proposed periodically.

Off By 26 Seconds

Although the Gregorian system synced the calendar with the seasons, it was not perfect; it was, and still is, off by 26 seconds. Consequently, a discrepancy of several hours has arisen since the Gregorian calendar was introduced. By the year 4909, the calendar will be a full day ahead of the solar year.