How did the present-day calendar originate?

Answer 1

The ancient Romans used a complicated 12-month calendar somewhat similar to the modern calendar, but each year was based on the ascension of the current emperor, so there was no long-term continuity of years.
The Julian calendar, introduced by Julius Caesar in 46 BCE, was a reform of the old calendar, chosen after consultation with the astronomer Sosigenes of Alexandria. It has a 12-month year of 365 days, with a leap day added to February every four years. A later reform was to fix the year as counting from the reign of Diocletian.

In what is now 533 CE, the Roman abbot Dionysus Exiguus proposed a change to the year of the calendar. He knew that it was impossible to say when Jesus was born, but he knew, or thought he knew, when Herod died. So, he devised the new Christian calendar to begin on the year of Herod's death. He based the date of Herod's death on the reign of the Roman emperor Augustus, but was unaware that Augustus only adopted that name four years after his reign began, going by his birth name of Octavius until then. So, Exiguus commenced his calendar just 4 years too late. This proposed new calendar was adopted in Western Europe after it was used by the Venerable Bede to date the events in his Ecclesiastical History of the English People, completed in 731.

The Julian calendar was still not exactly the same as the earth's tropical year, and seasonal drift continued to occur over long periods of time. To correct this error, the Gregorian calendar was proposed by the Aloysius Lilius and decreed by Pope Gregory XIII in 1582. The Gregorian calendar system dropped 10 days to bring the calendar back into synchronisation with the seasons and adopted the rule that every year exactly divisible by four is a leap year, except for years that are exactly divisible by 100, but that years that are exactly divisible by 400 are still leap years.