Gregorian Calendar: History, Leap Year Rules & How It Works

The Gregorian calendar is a solar calendar system introduced on Friday, October 15, 1582, by Pope Gregory XIII to reform the Julian calendar.

It has 12 months, 365 days in a common year, and 366 days in a leap year, with an average year length of 365.2425 days.

The Gregorian calendar is the civil calendar used by 195+ countries and the basis for ISO 8601, the international date standard. It was not created from scratch — it was a precise mathematical correction of a 1,500-year-old system that had drifted 10 full days out of alignment with the sun.

The most striking fact about its introduction: ten days were permanently deleted from history. Thursday, October 4, 1582, was followed immediately by Friday, October 15, 1582. No one who lived through that transition experienced those dates. They simply ceased to exist.

This page covers the Gregorian calendar’s origin, structure, leap year algorithm, accuracy, global adoption timeline, comparisons to the Julian, Islamic, Hebrew, Ethiopian, and Chinese calendars, its living financial legacy in the UK tax year, its role in computing, and the countries that still do not use it.

What Is the Gregorian Calendar?

The Gregorian calendar is a solar calendar — a system that aligns the count of days with Earth’s orbit around the Sun.

It replaced the Julian calendar across Catholic Europe beginning in 1582 and became the international civil standard across the following four centuries.

Key Facts at a Glance

Attribute	Value
Calendar type	Solar (proleptic Gregorian for dates before 1582)
Introduced	Friday, October 15, 1582
Decreed by	Pope Gregory XIII via Inter gravissimas
Designed by	Aloysius Lilius (astronomer), Christopher Clavius (mathematician)
Replaced	Julian calendar (introduced 46 BC by Julius Caesar)
Total months	12
Common year	365 days
Leap year	366 days
Average year length	365.2425 days
Annual error vs. tropical year	~27 seconds
Cumulative error (1 full day)	Every ~3,030 years
International standard	ISO 8601
Countries using it as civil calendar	195+

Why Is It Called the Gregorian Calendar?

The calendar is named after Pope Gregory XIII, who issued the papal bull Inter gravissimas on Wednesday, February 24, 1582. The name distinguishes it from the Julian calendar, which was named after Julius Caesar.

The mathematical architecture of the reform, however, was not Gregory’s work — it was designed by Aloysius Lilius, an Italian physician and astronomer, and promoted by Jesuit mathematician Christopher Clavius, who defended the reform against critics in the years following its adoption.

The calendar is not named after a single inventor. It represents a collaborative ecclesiastical and scientific project driven by a specific problem: the drift of Easter away from the spring equinox.

What the Gregorian Calendar Is Not

The Gregorian calendar is not a lunar calendar. It does not track the phases of the Moon. It is not the same as the Julian calendar, which it replaced, and it is not identical to the proleptic Gregorian calendar, which extends the same leap year rules backward mathematically to dates before 1582.

These distinctions matter particularly in genealogical research, historical scholarship, and computing.

History of the Gregorian Calendar

The Problem with the Julian Calendar

Julius Caesar introduced the Julian calendar in 46 BC. Its foundational assumption was that a solar year lasted exactly 365.25 days — handled by adding one leap day every four years. The actual mean tropical year (the time between successive vernal equinoxes) is approximately 365.24219 days. That difference is 11 minutes and 14 seconds per year.

Small as it sounds, this error accumulates. Over 128 years, it produces a 1-day drift. Over the 1,257 years between Caesar’s reform and the 1582 correction, it produced a drift of approximately 10 days.

By the 1570s, the vernal equinox was falling around March 11 instead of March 21. This was not merely an astronomical curiosity. The date of Easter in the Western Church is calculated relative to the spring equinox (the First Council of Nicaea in AD 325 had anchored Easter to March 21).

A drifting equinox meant Easter itself was drifting — which the Church considered both liturgically incorrect and administratively untenable.

Pope Gregory XIII and the Papal Bull Inter gravissimas

Pope Gregory XIII issued Inter gravissimas (“Among the most serious tasks”) on Wednesday, February 24, 1582. The bull mandated two changes:

1. A one-time correction: Skip 10 days to restore the equinox to March 21. Thursday, October 4, 1582, was followed immediately by Friday, October 15, 1582.
2. A new leap year rule: Modify the Julian system to prevent future drift.

The reform took effect in Catholic countries — Italy, Spain, Portugal, Poland, and France — in October 1582. Protestant and Orthodox nations resisted for decades to centuries, largely on political and religious grounds.

The Missing 10 Days of October 1582

What Actually Happened on Friday, October 15, 1582?

October 5 through October 14, 1582, do not exist in the Gregorian calendar. These dates were deleted by decree to resynchronize the calendar with the tropical year.

The October 1582 calendar is one of the most searched historical artifacts online. The virality is understandable — it is one of the few verifiable instances in recorded history where a government-mandated policy eliminated a block of time.

A common misconception is that people “lost” ten days of their lives. They did not. The adjustment was administrative, not biological. Rents, wages, and legal contracts were extended proportionally.

The Calendar (New Style) Act, in various forms, included explicit provisions to prevent creditors from demanding payments as if those days had elapsed. People continued to age normally. What changed was only the label assigned to each date.

The October 1582 calendar looked like this in affected countries:

Sunday	Monday	Tuesday	Wednesday	Thursday	Friday	Saturday
	1	2	3	4	15	16
17	18	19	20	21	22	23
24	25	26	27	28	29	30
31

Days 5–14 do not appear. Thursday, October 4, was followed by Friday, October 15.

Global Adoption Timeline

The Gregorian calendar was not adopted uniformly. Catholic countries adopted it immediately. Protestant nations resisted, and Orthodox countries held out for centuries. The following table documents the full adoption record:

Country / Region	Year Adopted	Days Skipped	Notes
Italy, Spain, Portugal, Poland	1582	10	First adopters
France	1582	10	December in France
Prussia, parts of Netherlands	1582–1583	10	Phased
Hungary	1587	10
German Catholic states	1583–1584	10
German Protestant states	1700	11
Denmark, Norway	1700	11
Great Britain and colonies	Wednesday, September 2 → Thursday, September 14, 1752	11	Calendar (New Style) Act 1750
Sweden	1753	11	After a botched gradual transition 1700–1712
Japan	1873	—	Meiji government decree
China	1912	—	Republic of China; wider use from 1929
Bulgaria	1916	13
Russia	1918	13	Soviet decree; February 1 OS became February 14 NS
Serbia, Romania	1919	13	Post–WWI
Greece	1923	13	Last European country
Turkey	1926	—	Ottoman to Republic transition
Saudi Arabia	2016	—	Switched for civil use; Umm al-Qura calendar retained for religious use
Ethiopia	Not adopted	—	Still uses the Ethiopian (Ge’ez) calendar

The gap between 1582 and 1923 is 341 years. This means that for over three centuries, two calendar systems ran simultaneously in Europe, creating “double dating” problems in legal documents, international trade, and historical records.

Double Dating and Old Style / New Style Notation

Between 1582 and 1923, historians and legal scribes in countries that had not yet adopted the Gregorian calendar used a dual notation to avoid ambiguity. A date written as “11th February 1731/32” indicated:

• Julian (Old Style) date: February 11, 1731
• Gregorian (New Style) equivalent: February 22, 1732

George Washington’s birthday illustrates this directly. He was born on February 11, 1731 (Old Style, Julian calendar, as recorded in Virginia).

After Britain adopted the Gregorian calendar in 1752, this date converts to February 22, 1732 (New Style, Gregorian). Washington himself, after 1752, celebrated his birthday on February 22, which is why that date is commemorated today.

The year discrepancy (1731 vs. 1732) arises because England’s legal year began on Lady Day, March 25, not January 1. Under the old system, February 1731 was still in the year 1731. Under the new system, it fell in 1732.

How the Gregorian Calendar Works

The 12 Months and Their Lengths

The Gregorian calendar has 12 months with irregular lengths, all inherited from the Roman calendar:

Month	Days	Name Origin
January	31	Janus (Roman god of beginnings)
February	28 / 29	Februa (Roman purification festival)
March	31	Mars (Roman god of war)
April	30	Possibly from Latin aperire (to open)
May	31	Maia (Roman goddess)
June	30	Juno (Roman goddess)
July	31	Julius Caesar
August	31	Augustus Caesar
September	30	Septem (seven) — 9th month under modern system
October	31	Octo (eight) — 10th month under modern system
November	30	Novem (nine) — 11th month under modern system
December	31	Decem (ten) — 12th month under modern system

September through December carry names derived from Latin numbers 7 through 10 because the original Roman calendar, attributed to King Romulus, had only 10 months and began in March.

January and February were added later, around 713 BC, under King Numa Pompilius, which shifted the numbered months two positions forward without renaming them.

February is the shortest month because the Romans considered even numbers unlucky. When Julius Caesar and later Augustus Caesar each claimed a month (July and August), they each required 31 days, and the surplus was taken from February.

Gregorian Leap Year Algorithm

The Three-Part Rule

The Gregorian leap year rule corrects the over-accumulation of days that results from the simpler Julian rule of “every four years.” The three-part algorithm is:

1. A year divisible by 4 is a leap year. Example: 2024 ÷ 4 = 506 → leap year ✓
2. Exception: A year divisible by 100 is not a leap year. Example: 1900 ÷ 100 = 19 → not a leap year ✗
3. Exception to the exception: A year divisible by 400 is a leap year. Example: 2000 ÷ 400 = 5 → leap year ✓

This algorithm produces 97 leap years per 400-year cycle, yielding an average year length of 365.2425 days. The Julian calendar, with its simpler “every 4 years” rule, produces 100 leap years per 400-year cycle — 3 too many.

Leap Year Application Examples

Year	Divisible by 4?	Divisible by 100?	Divisible by 400?	Leap Year?
2024	Yes	No	No	Yes
2025	No	—	—	No
2026	No	—	—	No
2028	Yes	No	No	Yes
1900	Yes	Yes	No	No
2000	Yes	Yes	Yes	Yes
2100	Yes	Yes	No	No
2400	Yes	Yes	Yes	Yes

The year 2026 is not a leap year. The next leap year is 2028.

The leap day, February 29, is also called an intercalary day or, in Latin tradition, a bissextile day — from the Roman practice of doubling the sixth day before the Calends of March (bis sextus, meaning “twice sixth”).

How Accurate Is the Gregorian Calendar?

The Gregorian calendar’s average year length of 365.2425 days compares to the mean tropical year of approximately 365.24219 days.

The annual error is approximately 27 seconds. This accumulates to one full day every ~3,030 years. At that rate, the Gregorian calendar will not require correction until approximately the year 4909.

For context on accuracy rankings among major calendar systems:

Calendar	Average Year Length (days)	Error vs. Tropical Year	Days off in 1,000 years
Persian Solar Hijri (Iran)	365.24242	~0.00022 days/year	~0.22
Gregorian	365.2425	~0.00031 days/year	~0.31
Julian	365.25	~0.0078 days/year	~7.8

The Persian Solar Hijri calendar (used in Iran and Afghanistan) is more astronomically accurate than the Gregorian calendar.

Its leap year cycle is determined by direct astronomical calculation rather than a fixed arithmetic rule, resulting in an error of approximately 1 day every 130,000 years versus the Gregorian calendar’s 1 day every ~3,030 years.

Gregorian Calendar vs. Other Calendar Systems

Calendars divide into three structural types: solar (aligned to the Sun), lunar (aligned to the Moon), and lunisolar (aligned to both). The Gregorian calendar is purely solar.

Gregorian Calendar vs. Julian Calendar

Feature	Julian Calendar	Gregorian Calendar
Introduced	46 BC by Julius Caesar	Friday, October 15, 1582
Leap year rule	Every year divisible by 4	Divisible by 4, except centuries, except centuries divisible by 400
Average year length	365.25 days	365.2425 days
Error vs. tropical year	~11 min 14 sec/year	~27 sec/year
Current drift from Gregorian	13 days behind	—
Used today by	Eastern Orthodox churches (liturgically)	195+ countries (civil use)

The Julian calendar is currently 13 days behind the Gregorian calendar. This gap was 10 days in 1582, and it grows by 3 days every 400 years because century years (1700, 1800, 1900, 2100) are not leap years in the Gregorian system but are in the Julian system.

This 13-day gap directly explains several historical and religious puzzles:

• Orthodox Christmas falls on January 7 (Gregorian) because it is December 25 on the Julian calendar, and December 25 Julian = January 7 Gregorian (currently).
• The October Revolution of 1917 is named for the month it occurred in on the Julian calendar. On the Gregorian calendar, it took place in November, which is why it is sometimes called the November Revolution in Western sources.
• Russia’s adoption in 1918: The day after January 31, 1918 (Julian) became February 14, 1918 (Gregorian) — a 13-day jump.

Gregorian Calendar vs. Islamic (Hijri) Calendar

The Islamic (Hijri) calendar is a purely lunar calendar. It has 12 months, each beginning with the sighting of the crescent moon. A Hijri year contains either 354 or 355 days — approximately 11 days shorter than a Gregorian year.

Feature	Islamic (Hijri) Calendar	Gregorian Calendar
Type	Lunar	Solar
Months per year	12	12
Days per year	354 or 355	365 or 366
Year 1 epoch	Hijra of the Prophet Muhammad, July 16, 622 AD (Gregorian)	January 1 of year 1 (proleptic)
Current year (as of 2026)	1447 AH	2026 CE
Drift vs. Gregorian	~11 days earlier per year	Fixed to solar year
Ramadan	Moves through all Gregorian seasons over ~33 years	—

Because the Hijri calendar is shorter than the solar year, it completes a full cycle through all Gregorian seasons in approximately 33 years.

This is why Ramadan falls in different Gregorian months each year — it is not a scheduling inconsistency, but a structural feature of the lunar system.

Conversion between Hijri and Gregorian dates requires formula-based calculation. There is no fixed offset because the relationship between the two systems shifts continuously.

Gregorian Calendar vs. Hebrew Calendar

The Hebrew calendar is lunisolar. It follows lunar months but periodically inserts a 13th month (Adar I) to keep the calendar aligned with the solar year. This occurs 7 times in every 19-year cycle, a structure known as the Metonic cycle.

Feature	Hebrew Calendar	Gregorian Calendar
Type	Lunisolar	Solar
Months per year	12 (or 13 in leap years)	12
Days per year	353–355 (common) / 383–385 (leap)	365 or 366
Year 1 epoch	Anno Mundi (year of creation), calculated as 3761 BC	Year 1 CE (proleptic)
Current year (as of 2026)	5786 AM	2026 CE
Primary use	Jewish religious observances (Rosh Hashanah, Yom Kippur, Passover, Shabbat)	Civil and international standard

The 19-year Metonic cycle in the Hebrew calendar means it re-synchronizes with the solar year precisely every 19 years, keeping major festivals in their seasonal positions. Passover, for example, always falls in spring in the Northern Hemisphere.

Gregorian Calendar vs. Ethiopian Calendar

Ethiopia uses the Ethiopian calendar (also called the Ge’ez calendar or Alexandrian calendar), which descends from the ancient Coptic calendar tradition. It has 13 months: 12 months of 30 days each, and a 13th month called Pagume containing 5 days (or 6 days in leap years).

Feature	Ethiopian Calendar	Gregorian Calendar
Type	Solar (Alexandrian tradition)	Solar
Months per year	13	12
Days per year	365 or 366	365 or 366
Current year (as of 2026)	2018 EC	2026 CE
Gap from Gregorian	7–8 years behind	—
New Year (Enkutatash)	September 11 or 12 (Gregorian)	January 1

Ethiopia celebrated its year 2000 in September 2007 on the Gregorian calendar. The ~7-to-8 year difference arises from a discrepancy in the calculation of the Anno Domini era, specifically from differing estimates of the birth of Jesus Christ used by the Alexandrian church versus the Roman church.

Gregorian Calendar vs. Chinese Lunisolar Calendar

The Chinese lunisolar calendar combines lunar months with solar terms to keep festivals aligned with the seasons. It is used primarily for traditional holidays, not civil administration (China uses the Gregorian calendar for civil purposes).

Feature	Chinese Lunisolar Calendar	Gregorian Calendar
Type	Lunisolar	Solar
Months per year	12 (or 13 in leap years)	12
New Year range on Gregorian	January 21 – February 20	January 1 (fixed)
Primary use	Traditional festivals (Chinese New Year, Mid-Autumn Festival, Qingming)	Civil standard globally

Chinese New Year does not have a fixed Gregorian date because the Chinese calendar inserts a leap month approximately every 3 years to prevent seasonal drift.

The leap month is inserted in the position where the sun does not pass through a major solar term, determined by precise astronomical calculation.

The Gregorian Calendar and the Eastern Orthodox Church

The Eastern Orthodox Church never adopted the Gregorian calendar for its liturgical calendar. This is not a matter of oversight — it is a deliberate theological and canonical position, most formally articulated in the Pan-Orthodox Congress of Constantinople in 1923.

The Julian calendar currently runs 13 days behind the Gregorian calendar. Because Orthodox churches that follow the Julian calendar observe December 25 (Julian) as Christmas, this date falls on January 7 (Gregorian) — which is why Orthodox Christmas is observed on January 7 in countries including Russia, Serbia, Ethiopia, and others.

Not all Orthodox churches follow the same system. In 1923, the Serbian astronomer Milutin Milanković (known for the Milanković cycles in climate science) proposed a Revised Julian Calendar, which aligns with the Gregorian calendar for fixed feasts but retains the Julian Paschalion (Easter calculation).

Several Orthodox churches — including the Greek Orthodox, Romanian Orthodox, Bulgarian Orthodox, and Antiochian Orthodox — adopted this Revised Julian Calendar. As a result, these churches celebrate Christmas on December 25 (which aligns with December 25 on the Gregorian calendar) but calculate Easter by the Julian method.

Churches that still follow the unreformed Julian calendar for all purposes include the Russian Orthodox Church, the Serbian Orthodox Church, the Georgian Orthodox Church, and the Jerusalem Patriarchate.

Orthodox Easter (Pascha) differs from Western Easter not only because of the Julian/Gregorian calendar difference, but also because of a separate rule: Orthodox Easter must fall after the Jewish Passover.

Western Christianity dropped this requirement at the Council of Nicaea. As a result, the two Easters can coincide in some years and diverge by up to five weeks in others.

The UK Tax Year and the Financial Legacy of 1752

Why the British Tax Year Starts on April 6

The British tax year starts on April 6 directly because of the 1752 Gregorian calendar adoption. This is one of the most concrete examples of a 16th-century papal reform still governing modern financial infrastructure.

The mechanism works as follows:

Under the Julian calendar, the English legal New Year began on Lady Day, March 25 — one of the four traditional Quarter Days. This was the date on which annual rents, wages, and tax periods were calculated.

When Britain adopted the Gregorian calendar on Thursday, September 14, 1752 (skipping from Wednesday, September 2 to Thursday, September 14 — 11 days), the Treasury faced a mathematical problem.

If the tax year ended on March 25 as usual, the government would collect tax on only 354 days instead of 365 — an 11-day shortfall in revenue.

The solution was to extend the tax year-end date by 11 days:

Step	Date	Rationale
Original tax year end	March 25	Lady Day, Julian New Year
+11 days (1752 correction)	April 5	Full 365-day tax cycle preserved
+1 day (1800 correction)	April 6	1800 was a Julian leap year but NOT a Gregorian leap year; one more day added
Current UK tax year start	April 6	Unchanged since 1800

The 1800 correction is frequently omitted in historical accounts. Because 1800 was a leap year under the Julian calendar (divisible by 4) but not under the Gregorian calendar (divisible by 100 but not 400), one additional day was inserted, pushing the tax year start from April 5 to April 6.

There was no equivalent correction in 1900 because 1900 was a non-leap year under both systems.

The UK financial year has run from April 6 to April 5 every year since 1800, solely as a consequence of two calendar switches 170 years apart. No other major economy uses this start date, and the reason is not found in tax policy — it is found in 16th-century astronomy.

The Gregorian Calendar in Computing and Programming

Java’s GregorianCalendar Class

Java’s GregorianCalendar class is a legacy implementation superseded by the java.time package introduced in Java 8 (released March 18, 2014). The class is still present in the JDK but is classified as legacy in practice by the Java documentation, which recommends java.time for new development.

The GregorianCalendar class is a concrete subclass of java.util.Calendar. It implements the standard Gregorian calendar and can also switch to the Julian calendar for dates before a configurable cutover date (by default, Friday, October 15, 1582 — the Gregorian introduction date).

Modern replacements in the java.time package include:

Legacy Class	Modern Replacement	Notes
java.util.GregorianCalendar	java.time.LocalDate	Date without time or timezone
java.util.GregorianCalendar	java.time.ZonedDateTime	Date-time with timezone
java.util.Date	java.time.Instant	Machine timestamp (Unix epoch)
java.util.Calendar.getInstance()	java.time.LocalDate.now()	Current date

The Proleptic Gregorian Calendar in Computing

The proleptic Gregorian calendar extends Gregorian leap year rules backward to dates before October 15, 1582 — including dates before the calendar existed.

ISO 8601 specifies the use of the proleptic Gregorian calendar for all dates, which is why computing systems can represent dates like January 1, 45 BC (Gregorian) without inconsistency.

This has three important implications:

1. Year Zero exists in the proleptic Gregorian calendar. Year 0 corresponds to 1 BC in traditional historical notation. Historians typically use the notation “1 BC” and skip year 0; astronomers and ISO 8601 use year 0. Software systems differ in their implementation, which is a documented source of off-by-one errors in date calculations.
2. Unix time does not use the Gregorian calendar directly. Unix time counts seconds elapsed since the Unix Epoch, defined as Thursday, January 1, 1970, 00:00:00 UTC. Conversion to a Gregorian date requires accounting for leap years and leap seconds.
3. Leap seconds are not part of the Gregorian calendar. The Gregorian calendar governs the count of days. Leap seconds are adjustments to Coordinated Universal Time (UTC) made by the International Earth Rotation and Reference Systems Service (IERS) to account for irregularities in Earth’s rotation. The Gregorian calendar will not need modification because of leap seconds.

Countries That Do Not Use the Gregorian Calendar

Several countries maintain official calendars other than the Gregorian calendar, or use dual systems in which the Gregorian calendar serves civil functions and a traditional calendar governs religious or cultural life.

Country	Official Calendar	Current Year (as of 2026)	Notes
Ethiopia	Ethiopian (Ge’ez) Calendar	2018 EC	7–8 years behind Gregorian; 13 months
Iran	Solar Hijri (Persian) Calendar	1404 SH	Most astronomically accurate solar calendar
Afghanistan	Solar Hijri Calendar	1404 SH	Same system as Iran
Nepal	Vikram Samvat	2082 BS	~56.7 years ahead of Gregorian
Israel	Hebrew + Gregorian (dual)	5786 AM	Hebrew for religious use; Gregorian for civil use
Saudi Arabia	Umm al-Qura + Gregorian (dual)	1447 AH	Switched to Gregorian for civil/government use in 2016
India	Saka (National Calendar) + Gregorian	1948 Saka	Gregorian dominates official and business use
Thailand	Thai Solar Calendar	2569 BE	Based on the Buddhist Era; Gregorian used in international contexts
Ethiopia, Eritrea (Coptic communities)	Coptic Calendar	1742 AM	Liturgical use within Coptic Christian communities

Ethiopia is the most frequently cited case. The country celebrates its new year (Enkutatash) on September 11 or September 12 of the Gregorian calendar. When the Gregorian calendar entered the year 2000 on January 1, 2000, Ethiopia did not reach the year 2000 until September 12, 2007.

Criticisms and Proposed Alternatives

Structural Weaknesses of the Gregorian Calendar

The Gregorian calendar has several structural irregularities that complicate scheduling, accounting, and date arithmetic:

• Irregular month lengths (28, 29, 30, or 31 days) make monthly comparisons inconsistent.
• The year does not begin at an astronomically defined event. January 1 does not correspond to a solstice, equinox, or perihelion.
• The weekday of any given date shifts every year, which means that a date like March 15 falls on a different weekday each year and cannot be relied upon for recurring weekly scheduling without a lookup.
• The year cannot be evenly divided into equal quarters — Q1 has 90 or 91 days, Q2 has 91, Q3 has 92, and Q4 has 92, creating discrepancies in financial reporting.
• The accumulated error of ~27 seconds per year will require correction in approximately 2,882 years (from 2026), around the year 4909.

The International Fixed Calendar

The International Fixed Calendar, also called the Cotsworth plan, was proposed by Moses B. Cotsworth in 1902. Its structure:

• 13 months of exactly 28 days (4 weeks each)
• 1 intercalary “Year Day” outside any month, at year-end
• 1 intercalary “Leap Day” outside any month, inserted after June 28 in leap years
• Every date falls on the same weekday every year (e.g., the 1st of every month is always Sunday)

The International Fixed Calendar was adopted internally by the Eastman Kodak Company and used in its accounting for decades, beginning in the 1920s. It was presented to the League of Nations in the 1920s and to the United Nations in the 1930s.

It was rejected primarily because a 13-month calendar disrupts the 7-day weekly cycle, which interferes with Sabbath observance in Judaism, Christianity, and Islam — all of which require an unbroken 7-day cycle.

The World Calendar

The World Calendar was proposed by Elisabeth Achelis in 1930 and promoted before the United Nations through the 1950s. Its structure:

• 12 months in 4 identical quarters
• Each quarter has 91 days (13 weeks exactly): two months of 30 days + one month of 31 days
• 1 “Worldsday” (W) added at year-end, outside the weekly cycle
• 1 “Leapyear Day” added after June 30 in leap years

Like the International Fixed Calendar, the World Calendar was rejected at the United Nations primarily due to religious objections. Any calendar that inserts a day outside the 7-day week breaks the continuous cycle of the Sabbath.

Frequently Asked Questions About the Gregorian Calendar

Gregorian Calendar Conversion Reference

Calendar conversion requires dedicated calculation tools because the relationships between calendar systems are not based on fixed offsets. The following table summarizes the conversion complexity for each major system:

Source Calendar	Target: Gregorian	Method	Complexity
Julian	Gregorian	Add 13 days (for dates after March 1, 1900)	Low — fixed offset per century
Islamic (Hijri)	Gregorian	Formula-based (lunar months vary)	High — no fixed offset
Hebrew	Gregorian	Metonic cycle calculation	High — lunisolar, requires tables
Ethiopian	Gregorian	Add ~7–8 years + month/day offset	Medium — systematic but requires rule
Persian Solar Hijri	Gregorian	Astronomical calculation or lookup	Medium-High
Chinese Lunisolar	Gregorian	Astronomical calculation (solar terms)	High

For bidirectional conversion between the Gregorian calendar and the Islamic (Hijri), Hebrew, Ethiopian, Persian, or Julian calendars, formula-based calculation or verified algorithmic tools are required. No manual shortcut exists for Islamic or Hebrew conversions because both systems use variable-length months determined by lunar cycles or rabbinical calendar rules.

Five Facts About the Gregorian Calendar

The following are five data points that distinguish this resource from standard encyclopedia treatments:

1. The UK’s April 6 tax year is a direct fiscal consequence of the 1752 calendar switch — not a standalone policy decision. The 11-day skip in September 1752 was extended by one more day in 1800 due to the Gregorian century-year rule. The Treasury has not corrected this since.
2. The Persian Solar Hijri calendar is more accurate than the Gregorian calendar — its error is approximately 1 day per 130,000 years versus 1 day per ~3,030 years for the Gregorian system.
3. George Washington has two verified birthdays: February 11, 1731 (Old Style, Julian) and February 22, 1732 (New Style, Gregorian). Both are based on the same recorded birth event.
4. Java’s GregorianCalendar class was effectively deprecated in practice with Java 8 (2014), but remains in the JDK, which is why developer searches for it have increased, not because the class is new, but because codebases written before 2014 still use it.
5. The Gregorian calendar will not need correction until approximately the year 4909 — 2,883 years from 2026 — based on its current rate of drift against the mean tropical year.

The Gregorian calendar remains the backbone of global civil timekeeping, international commerce, computing infrastructure, and legal systems.

Its 1582 design has proven robust enough to govern a planet’s scheduling for over 440 years — and, barring deliberate reform, will continue to do so for approximately three millennia more.