Gregorian calendar introduced The Gregorian calendar was introduced by Pope Gregory XIII in 1582 to address the cumulative inaccuracies of the Julian calendar. The Julian calendar was based on a Roman Republican dating system introduced by Julius Caesar in 46 BC. Under this system, the legal new year began on 25 March and the formula used to calculate leap years produced one such year every four years. This was far too often. By the 1500s, the Julian calendar became 11 days out of sync and hopelessly inaccurate. It did not properly reflect the actual time it takes the earth to orbit once around the sun. To create the Gregorian calendar, the formula for calculating leap years was changed and the start of the legal new year moved to 1 January. The calendar uses a 365-day common year, divided into twelve months of irregular lengths. Eleven months have 30 or 31 days, while February has 28 days during the common year. Nearly every four years is a leap year. Such years have an extended length of 366 days due to the addition of one day, namely 29 February. The introduction of the Gregorian calendar allowed for realignment events such as equinoxes. Note that the calendar disc on David Ramsay's King James Portrait Watch, made around 1618, is designed to count the days according to the Gregorian calendar. At the time of the new calendar’s introduction, it was adopted by Catholic countries. Protestant countries continued to use the Julian calendar until the 1700s, and England did not adopt the Gregorian calendar until 1752. The Gregorian calendar is today's internationally accepted civil calendar and is the most widely used calendar in the world. Image Credit Eeuwigdurende Gregoriaanse kalender met de vier seizoenen, Johann Sadeler (I), 159. (Gregorian perpetual calendar, with the Four Seasons), 1595, engraving on paper, 317 × 242 mm; Amsterdam, Rijksmuseum, RP-P-OB-7498. Photo: Rijksmuseum CC0 Public domain
