How many days are in February?

1.1 Overview and Standard Duration

February is the second month of the year in both the Julian and Gregorian calendars. Unlike the other eleven months, which consist of either thirty or thirty-one days, February stands out as the sole month with fewer than thirty days. Under normal astronomical and calendar calculations, a standard year—known as a common year—assigns exactly twenty-eight days to February. This yields a total annual count of three hundred sixty-five days. Because of this reduced length, February can pass entirely without a full moon in certain years, a phenomenon colloquially referred to as a black moon.^{[1]2월은 날수가 적어 보름달이 한 번도 뜨지 않는 달이 되기도 한다. 이때 발생하는 보름달의 부재 현상을 '블랙 문'이라고 부른다.}

1.2 The Leap Year System

The variation between twenty-eight and twenty-nine days exists to synchronize the human calendar year with the astronomical solar year. The time required for the Earth to complete one full orbit around the Sun is approximately three hundred sixty-five days and six hours. If every calendar year strictly contained three hundred sixty-five days, the calendar would drift away from the actual solar seasons by roughly six hours each year, leading to a shift of nearly twenty-four days over the span of a century.

To compensate for this fractional difference, an extra day is introduced into the calendar once every four years. This extra day, known as a leap day, is added directly to February, increasing its length from twenty-eight to twenty-nine days. A year containing this additional day is designated as a leap year and totals three hundred sixty-six days.

The rules governing leap years in the modern Gregorian calendar dictate that a year is a leap year if its numerical designation is evenly divisible by four. However, century years present an exception to prevent overcorrection. A century year is only considered a leap year if it is also evenly divisible by four hundred. Consequently, years such as sixteen hundred and two thousand featured a twenty-nine-day February, whereas years such as seventeen hundred, eighteen hundred, and nineteen hundred contained only twenty-eight days.

1.3 Historical Origin of the Short Length

The peculiar length of February originates from the ancient Roman calendar established by Romulus, which originally contained only ten months and excluded the winter period entirely.^{[2]고대 로마의 초기 달력에는 겨울철 추위로 농사를 짓지 못하는 기간을 아예 달력에서 제외하여 10개 월만 존재했다.} When Numa Pompilius revised the calendar, he added January and February to cover the winter months. Roman superstition favored odd numbers for month lengths, leaving February as the final month of the calendar year at the time. As a result of balancing the remaining days to fit an annual lunar cycle, February was designated as an unlucky, even-numbered month containing twenty-eight days, dedicated to purification rituals known as Februa.

When Julius Caesar reformed the system into the solar Julian calendar, February retained its position as the shortest month, though its role as the final month shifted to January. The modern Gregorian calendar maintained this historical duration, preserving February's status as a variable month of twenty-eight or twenty-nine days.

1.4 Exceptional Historical Occurrences

Although February is strictly limited to twenty-eight or twenty-nine days in contemporary usage, rare historical anomalies have resulted in a thirty-day February. The most notable instance occurred in Sweden during the early eighteenth century. When transitioning from the Julian calendar to the Gregorian calendar, Sweden initially planned to omit leap days over a period of forty years. However, after failing to follow the schedule consistently, the Swedish calendar became unsynchronized with both systems. To rectify the confusion and return to the Julian system, two leap days were added to February in the year seventeen twelve, resulting in a unique February thirty.^{[3]스웨덴은 윤년을 빼먹는 실수를 수습하기 위해 1712년에 2월 30일을 실제로 만들어서 사용했다.}

Additionally, theoretical proposals for calendar reform, such as the Soviet revolutionary calendar tested in the early twentieth century, briefly envisioned months of uniform thirty-day lengths, though such implementations were either localized, temporary, or never fully realized in long-term global practice.

2.1 Leap Year Origin and Julian Calendar Reform

The insertion of a 29th day into February every four years originates from the establishment of the Julian calendar by Julius Caesar in 46 BCE. Prior to this reform, the Roman calendar operated on a 355-day lunar cycle, which required the periodic insertion of an entire intercalary month, *Mercedonius*, to realign the calendar with the solar year. Caesar, assisted by the Alexandrian astronomer Sosigenes, standardized the year to 365.25 days. To account for the fractional day, a single leap day was introduced every four years, permanently affixed to the shortest month, February.

2.2 Gregorian Reform and the Century Rule

While the Julian calendar assumed a solar year of exactly 365.25 days, the actual mean tropical year is approximately 365.2422 days. This minute variance of roughly 11 minutes per year accumulated over centuries, drifting from the astronomical seasons by about three days every four centuries. In 1582, Pope Gregory XIII promulgated the Gregorian calendar reform to correct this drift.^{[4]교황 그레고리우스 13세는 1582년 칙령 'Inter gravissimas'를 통해 그레고리력 개혁을 단행했다.} Under the new system, century years (years ending in "00") do not contain 29 days in February unless they are evenly divisible by 400. Consequently, the years 1700, 1800, and 1900 possessed 28 days in February, whereas the year 2000 possessed 29.

2.3 Historical Anomalies: February 30

Although theoretically impossible under standard solar calendar frameworks, February 30 has manifested in real-world administrative history.

• Swedish Empire (1712): The Swedish Empire planned to transition from the Julian calendar to the Gregorian calendar by omitting leap days from 1700 to 1740. However, due to the outbreak of the Great Northern War, the 1704 and 1708 leap days were mistakenly retained. To resolve the resulting chronological misalignment, King Charles XII ordered the empire to revert to the Julian calendar in 1712 by adding two leap days to February, creating a unique 30-day February for that year.
• Soviet Union (1930–1931): Following the introduction of the Soviet Revolutionary Calendar in 1929—which featured five-day work weeks (*pyatidnevka*) and 30-day months—February officially contained 30 days in both 1930 and 1931. The system proved economically impractical and was gradually phased out by late 1931.

2.4 Misconception of August Calendar Manipulation

A persistent historical myth claims that February originally possessed 29 days in common years, but lost a day to August during the reign of Augustus Caesar.^{[5]아우구스투스 황제가 자신의 달인 8월을 7월처럼 길게 만들려고 2월에서 하루를 빼앗아 갔다는 소문이 널리 퍼져 있으나, 이는 중세 학자 사크로보스코가 만든 가짜 뉴스다.} The narrative asserts that August (formerly *Sextilis*) had 30 days, but Augustus demanded his namesake month match the 31-day length of Julius Caesar's July, prompting Roman authorities to subtract one day from February and add it to August. Epigraphic and chronological records demonstrate this to be false; Sacrobosco’s 13th-century theory has been debunked by modern historians who confirmed that the lengths of the months were fixed in their current arrangement by the initial Julian reform in 46 BCE.

2.5 Legal Status of Leaplings

Individuals born on February 29 are colloquially termed "leaplings" or "leap year babies." The determination of their legal age in non-leap years varies significantly across international jurisdictions.

• Taiwan (Republic of China): Under Article 124 of the Civil Code, a leapling's legal birthday in a non-leap year falls officially on March 1.
• United Kingdom and the United States: Common law tradition generally dictates that a person born on February 29 legally advances in age on March 1 in non-leap years for statutory rights such as voting or purchasing age-restricted goods.
• Japan: Japanese law regarding age calculation (Act No. 50 of 1902) specifies that a person ages at the first moment (midnight) of the day preceding their date of birth. Consequently, a leapling legally ages at midnight on February 28 in non-leap years.^{[6]https://elaws.e-gov.go.jp/document?lawid=135AC0000000050 — 일본은 '연령 계산에 관한 법률'에 따라 탄생일 전날 자정에 나이를 먹도록 규정하여 2월 29일 생은 2월 28일 24시에 법적 나이가 올라간다.}

2.6 Astronomical Variance and Long-Term Horizon

The Earth's axial precession and tidal acceleration caused by lunar gravitational forces are gradually slowing the planet's rotational speed. As a result, the length of a solar day increases by approximately 1.7 milliseconds per century. Over profound geological epochs, this slowing will alter the ratio of days within a tropical year, eventually necessitating an overhaul of the 4-year intercalary cycle. Furthermore, the Gregorian calendar's rule structure leaves a residual error of approximately 1 day every 3,216 years, meaning the 400-year leap cycle will eventually require further correction to prevent February's length from decoupling from the vernal equinox.