Until 1582, Europe had used the Julian calendar, established by Julius Caesar, which over time had begun to prove outdated. The equinoxes and solstices occurred too early, a fact that had already been noticed in the Middle Ages. Moreover, the flaws in the calendar created problems for dating Christian feasts, an issue the Church was called to address. Several popes had already commissioned mathematicians and astronomers to solve the problem, and even at the Council of Trent the cardinals had promised to deal with the matter. However, due to the complexity and sensitivity of such a task, the definitive reform of the calendar only came under Pope Gregory XIII in 1582.

In the commission convened by the pope, the German Jesuit Christoph Clavius, professor of mathematics and teacher of figures such as Alessandro Valignano and Matteo Ricci, played a key role, as did the brothers Antonio and Luigi Lilio, professors of mathematics and astronomy. The pope presented the reform in the papal bull Inter Gravissimas, in which he described the changes concerning the frequency of leap years, the timing of the equinoxes and solstices, and the dating of feasts. In particular, the pontiff decreed that October 4 of that year would be followed immediately by October 15, to recover the days lost under the Julian calendar.

The new calendar was soon adopted throughout the Catholic world, while Protestant and Orthodox countries took much longer. At the end of the 1500s, hostility between Catholics and Protestants was at its height, and although Protestant astronomers such as Kepler and Tycho Brahe considered the new calendar favorably, many countries refused to adopt it, dismissing it as a “papist novelty.” Many were also convinced that it was a way for the bishop of Rome to distort the perception of time, preventing Christians from being prepared for the end of days, in line with the widespread belief that the pope was in fact the Antichrist. Not by chance, around that time the University of Tübingen, in Protestant Germany, labeled as “allies of the Antichrist” anyone who accepted the reformed calendar.

It was only in the mid-1700s, in a religiously calmer climate, that Protestant countries such as England and Sweden definitively accepted the Gregorian calendar. In Orthodox Russia, however, the tsar refused to abandon the Julian calendar until 1917, when the October Revolution imposed the adoption of the Gregorian system.