Short answers, pulled from the story.
The word calendar comes from the Latin term kalendae, which meant the first day of the month in ancient Rome. This root verb referred to the public announcement made when the new moon was first seen in the sky.
Julius Caesar reformed the Roman calendar in 46 BC to create the Julian system. His algorithm introduced a leap day every four years instead of relying on moon observation.
Pope Gregory XIII introduced the Gregorian calendar in 1582 to correct errors accumulating over centuries. Greece became the last European nation to adopt this reform in 1923.
The Islamic Hijri calendar consists of twelve lunar months totaling three hundred fifty-four or three hundred fifty-five days per year. Its epoch marks the Hijra event corresponding to AD 622.
Greece became the last European nation to adopt the Gregorian calendar reform in 1923. The Gregorian calendar serves as the de facto international standard for civil purposes almost everywhere today.