The History of the Gregorian Calendar: Why October 1582 Lost 10 Days

How a 16th-century papal reform corrected a 1,600-year-old drift in the Julian calendar, why Protestant nations resisted it for centuries, and how it became the global civil standard.

March 24, 2026 · 11 min read · Gabriel

Table of Contents

A calendar that drifted out of alignment

In February 1582, Pope Gregory XIII signed the papal bull Inter gravissimas and quietly fixed a problem that had bothered astronomers, farmers, and church officials for more than a thousand years. The Julian calendar, instituted by Julius Caesar in 45 BCE on the advice of the Alexandrian astronomer Sosigenes, assumed that a tropical year was exactly 365.25 days long. The actual length is roughly 365.2422 days. The difference is small — about 11 minutes and 14 seconds per year — but it accumulates. By the late 16th century the Julian calendar had drifted about 10 days ahead of the seasons, and the vernal equinox, which the Council of Nicaea in 325 CE had pegged to March 21 for the purpose of computing Easter, was actually falling on March 11.

The reform commission

The reform was not improvised. Gregory XIII inherited a project that had been discussed at the Council of Trent (1545–1563) and entrusted to a commission that included the Calabrian physician and astronomer Aloysius Lilius (Luigi Lilio) and the German Jesuit mathematician Christopher Clavius. Lilius proposed the elegant correction; Clavius worked out the technical details and defended the new system for decades against its critics. Lilius died before the reform was promulgated, but his manuscript Compendium novae rationis restituendi kalendarium formed the backbone of the new calendar.

The leap year rule, refined

The fix had two parts. The first was arithmetic: keep the four-year leap rule from the Julian calendar, but skip the leap day in years divisible by 100 — unless they are also divisible by 400. So 1600 and 2000 were leap years, but 1700, 1800, and 1900 were not. The result is an average calendar year of 365.2425 days, which differs from the true tropical year by only about 27 seconds.

October 1582: ten days that never existed

The second part was the dramatic one. To realign the calendar with the seasons, ten days had to be deleted outright. In the Catholic countries that adopted the reform immediately — Spain, Portugal, the Italian states, and the Polish-Lithuanian Commonwealth — Thursday October 4, 1582 was followed by Friday October 15, 1582. Contracts, rents, and saints' days that fell in the missing window had to be handled by special clauses.

Two centuries of uneven adoption

Because the reform came from Rome, Protestant and Orthodox states viewed it with suspicion. France adopted it in December 1582, but the Dutch provinces split: Holland and Zeeland switched in 1583, while the eastern provinces waited until 1700–1701. The German Protestant states held out until 1700, Great Britain and its colonies until 1752 (by which point eleven days had to be dropped), Sweden made an infamous botched attempt that left it with a unique February 30 in 1712, and Russia did not switch until 1918, after the Bolshevik revolution. That is why the "October Revolution" actually happened in November by the Gregorian count. Greece was the last European country to convert, in 1923.

Why the Eastern Orthodox churches partly held back

Several Orthodox churches still use the Julian calendar for liturgical purposes, which is why Orthodox Christmas falls on January 7 in the civil (Gregorian) calendar — that is December 25 on the Julian one. A revised Julian calendar adopted by some Orthodox churches in 1923 happens to agree with the Gregorian calendar until the year 2800, but the underlying rule is slightly different.

Why the Gregorian calendar won globally

The Gregorian calendar's worldwide spread had less to do with religion than with commerce, diplomacy, and railways. The 19th and early 20th centuries saw a wave of standardization — international postal treaties, telegraph networks, shipping schedules, the prime meridian conference of 1884 — that made a single civil calendar enormously useful. By the time the League of Nations was founded in 1920, the Gregorian calendar was the de facto international standard.

What the calendar still does not solve

The Gregorian calendar is excellent at tracking the tropical year, but it does not align with the lunar month, the week, or the quarters of a business year. February has 28 or 29 days; quarters have 90, 91, or 92. Reformers from Auguste Comte to the United Nations Committee on Calendar Reform in the 1950s have proposed alternatives, but none has gained traction.