GREGORIAN CALENDAR

PART OF SERIES, "TRACKING TIME: BEHIND THE CLOCK AND CALENDAR"

Author: Ken Klark L. Flores

Introduction

Our common and widely used calendar is using, "Gregorian Calendar", which is used in our everyday lives, even used it in earth-based missions and official communications in space, and often using UTC Time.

Origins

It is originated in February 1582, when Pope Gregory XIII issued a papal bull (Inter gravissimas) to reform the Julian calendar, which had become misaligned with the seasons. The reform, based on the work of physician Aloysius Lilius, corrected the Julian calendar's error in calculating the solar year by skipping 10 days and adjusting the rules for leap years. This new calendar was gradually adopted by different countries, with Catholic nations being the first to switch in 1582 (Julian & Gregorian Calendar Systems, 2025).

The calendar's design was primarily the work of Italian astronomer and physician Luigi Lilio (Aloysius Lilius), with assistance from Jesuit astronomer Christopher Clavius (villaludovisi, 2017). To realign the calendar with the seasons, ten days were skipped; in Catholic countries, Thursday, October 4, 1582, was followed immediately by Friday, October 15, 1582. The system of leap years was modified to prevent future drift. A year is a leap year if it is divisible by four, but centurial years (e.g., 1700, 1800, 1900) are only leap years if they are also exactly divisible by 400 (e.g., 1600, 2000). This makes the Gregorian year an average of 365.2425 days long, much closer to the actual solar year of approximately 365.2422 days. (Cobabe, S.; Hocken, n.d.)

Adoption

The process was piecemeal, largely due to political and religious resistance, especially from Protestant and Eastern Orthodox nations reluctant to adopt a papal initiative. Catholic countries, including Italy, Poland, Portugal, Spain, and most of France, adopted the new calendar immediately following Pope Gregory XIII's papal bull Inter gravissimas. And eventually becoming the international civil standard due to its superior accuracy over the Julian calendar (From Julian to Gregorian Calendar, 2019).

Non-European countries began adopting the calendar for civil use, such as Japan in 1873. It became the world's most widely used civil calendar due to its superior accuracy in aligning with the solar year. 

Invention vs. Reform

The Catholic Church did not invent the concept of a solar calendar system from scratch; rather, it was instrumental in the commissioning and implementation of the Gregorian calendar as a reform of the existing Julian calendar (Gregory XIII Reforms the Calendar | EBSCO, 2022).

Secular Utility vs Religious Doctrine

Arguments that Protestants should not use the Gregorian calendar because it is a product of the Catholic Church are based on a misunderstanding of both history and the calendar's function. The Gregorian calendar is a globally recognized civil standard, not a religious doctrine or an object of worship (CHRISTIANITY | Understanding the Gregorian Calendar, 2024).

The calendar is a system for measuring time and organizing civil life (work weeks, holidays, seasons, etc.) Using it is a practical convenience, similar to using the metric system, a specific type of plumbing, or navigating using maps developed by people of different faiths. It does not imply adherence to Catholic theology. Creating a new, separate calendar system for Protestants would be chaotic and impractical in a modern, interconnected world. There is no biblical mandate nor viable alternative for a specific civil calendar, and uniformity is crucial for global coordination. (Gregory XIII | Research Starters | EBSCO Research, 2022).

Conclusion

Using the Gregorian calendar is almost universally accepted among Protestants (and people of most other faiths worldwide) as a neutral, practical tool for organizing daily life, entirely separate from theological differences with the Catholic Church. 

Reference

Julian & Gregorian Calendar Systems (2025) | Overview & Differences - Video | Study.com. Study.com. https://study.com/academy/lesson/video/origins-of-the-julian-gregorian-calendars.html

‌villaludovisi. (2017, April 8). Pietro Gagliardi’s Rediscovered Gregorian Calendar Fresco: A Snapshot of Scientific History. Archivio Digitale Boncompagni Ludovisi. https://villaludovisi.org/2017/04/08/pietro-gagliardis-rediscovered-calendar-fresco-a-snapshot-of-scientific-history/

‌Cobabe, S. (2023). What Calendar Do We Use? | All about the Gregorian Calendar. Familysearch.org. https://www.familysearch.org/en/blog/what-calendar-do-we-use-gregorian

‌Hocken, V. (n.d.). The Gregorian Calendar. Www.timeanddate.com. https://www.timeanddate.com/calendar/gregorian-calendar.html

‌From Julian to Gregorian Calendar. (2019). Timeanddate.com. https://www.timeanddate.com/calendar/julian-gregorian-switch.html

‌Gregory XIII Reforms the Calendar | EBSCO. (2022). EBSCO Information Services, Inc. | Www.ebsco.com. https://www.ebsco.com/research-starters/history/gregory-xiii-reforms-calendar

‌CHRISTIANITY | Understanding the Gregorian calendar. (2024, June 26). Religious Affairs Office. https://ajuntament.barcelona.cat/oficina-afers-religiosos/en/blog/christianity-understanding-gregorian-calendar

‌Gregory XIII | Research Starters | EBSCO Research. (2022). EBSCO. https://www.ebsco.com/research-starters/history/gregory-xiii

‌