How many different calendar systems are still actively used around the world?

The answer

Over 40 calendar systems remain in active use worldwide. The Gregorian calendar, introduced by Pope Gregory XIII in 1582, is the international standard but coexists with many others. The Islamic (Hijri) calendar is purely lunar with 354-day years, used for religious observances by 1.8 billion Muslims. The Hebrew calendar combines lunar months with solar year adjustments and dates creation to 3761 BCE. Ethiopia uses the Ge'ez calendar, which is 7 to 8 years behind the Gregorian - Ethiopia celebrated its millennium in 2007. The Chinese lunisolar calendar determines traditional holidays for 1.4 billion people. Thailand officially uses the Buddhist Era, currently 543 years ahead. Iran's Solar Hijri calendar is one of the world's most astronomically accurate. India alone officially recognizes 4 national calendars. Japan uses both the Gregorian and imperial era system, currently in the Reiwa era since 2019.