Today, the vast majority of the world uses what is known as the Gregorian calendar, Named after Pope Gregory XIII, who introduced it in 1582. The Gregorian calendar replaced the Julian calendar, which had been the most used calendar in Europe until this point. The Gregorian calendar lasts for approximately 365.24 days; this means that most years have 365 days, with one extra day being added every fourth year, unless the year is divisible by 100 but not 400 (for example, the year 2000 was a leap year, whereas 2100 and 2200 will not be).
Ancient Calendars
Mesoamerican and Asian civilizations focused more on cycles of years (such as the Chinese Zodiac), rather than recording centuries or millennia, therefore it is sometimes difficult to translate older calendars into modern terms. For example, the Mayan calendar did not develop in the same way as the Eurasian calendars, and cannot be included with this data, as January 1, 2020 is written as '13.0.7.2.7'. However, historians agree that by 1CE (in the Gregorian calendar), most societies had realized that one year lasted for approximately 365.25 days, and most of the more modern calendars split each year into twelve months. The major differences between each calendar was the starting points of the year and months, with some beginning with the arrival of spring, therefore having no fixed date, and the addition of leap years varied across all civilizations.
Revolutionary changes
The most recent calendar included here is the French Revolutionary Calendar, which was an attempt to further abandon the 'Ancien Régime' by decimalizing time and restructuring the calendar into twelve months of thirty days, with five or six holiday days left over. The calendar became official in 1793, and was implemented across France and French-occupied areas of Europe, but it was then abandoned after just twelve years. In more recent times, there have been some campaigns to create a new, universal calendar, however the Gregorian calendar has proven again too difficult to replace, having itself become universally adopted, while the other surviving calendars are used mostly for ceremonial purposes.
