The Gregorian calendar was introduced by Pope Gregory XIII in 1582 as a reform of the earlier Julian calendar, but its detailed design was created by the Italian doctor and astronomer Aloysius (Luigi) Lilius.

Who ā€œinventedā€ it?

In everyday terms, people say Pope Gregory XIII ā€œinventedā€ the Gregorian calendar because he officially proclaimed and implemented the reform that now bears his name. However, the actual mathematical schemeā€”the leap-year rules and the lunar corrections for calculating Easterā€”was devised by Aloysius (Luigi) Lilius, whose proposal the papal commission adopted.

Quick historical scoop

  • The Gregorian calendar is a reform of the Julian calendar, which had been in use since the time of Julius Caesar.
  • Pope Gregory XIII issued the papal bull Inter gravissimas on 24 February 1582, formally instituting the new calendar.
  • To fix accumulated drift, several days were skipped in October 1582 so that the date of Easter aligned better with the spring equinox again.

What exactly changed?

  • The Julian calendar treated every year divisible by 4 as a leap year, which made the average year slightly too long.
  • Liliusā€™s system reduced leap years by making century years nonā€‘leap unless divisible by 400 (so 1600 and 2000 are leap years, but 1700, 1800, 1900 are not).
  • This adjustment brought the average year length to about 365.2425 days, much closer to the actual tropical year and slowed seasonal drift.

How it spread over time

  • Catholic countries such as Italy, Spain, and France adopted the Gregorian calendar in 1582, soon after the papal decree.
  • Many Protestant and Orthodox countries resisted at first and only transitioned in later centuries, but today it is the dominant civil calendar worldwide.
  • Modern international standards for dates (like those behind many computer systems) are built on this Gregorian framework.

Forum-style take

In modern discussions, when someone asks ā€œwho invented the Gregorian calendar,ā€ the nuanced answer is:

  • Official inventor/introducer: Pope Gregory XIII (he ordered and promulgated it).
  • Scientific architect: Aloysius (Luigi) Lilius, whose calculations and proposal shaped the leap-year and Easter rules that define the calendar.

TL;DR: The Gregorian calendar is named for Pope Gregory XIII, who introduced it in 1582, but the underlying system was invented by the scholar Aloysius (Luigi) Lilius.

Information gathered from public forums or data available on the internet and portrayed here.