How Many Days Are In A Year?

A common year on the Gregorian calendar has 365 days; a leap year has 366. The extra day, 29 February, is inserted on years divisible by 4 - except for century years not divisible by 400. The full rule, ratified by Pope Gregory XIII's bull Inter gravissimas on 24 February 1582, can be written in three lines: divisible by 4 -> leap; divisible by 100 -> not leap; divisible by 400 -> leap (overriding the previous rule). So 1700, 1800, 1900 are common years, but 1600 and 2000 are leap.

Before the Gregorian reform, Europe used the Julian calendar, introduced by Julius Caesar in 46 BCE on the advice of the Alexandrian astronomer Sosigenes. The Julian year was 365.25 days exactly, inserting one leap day every four years without exception. This overestimated the true tropical year (365.2422 days) by 11 minutes 14 seconds per year - a drift of about three days every 400 years. By the late 16th century the Julian calendar had drifted ten days behind the equinox, so Pope Gregory XIII dropped 10 days from October 1582 (4 October 1582 was followed by 15 October 1582) and tightened the leap rule to the modern Gregorian formula.

The Gregorian calendar gives an average year length of (303 × 365 + 97 × 366) / 400 = 365.2425 days, which differs from the true tropical year by only 26.8 seconds per year. That works out to a drift of one day in about 3,236 years. The next calendar reform, if ever needed, will not arrive until well after 5000 CE. NIST in Boulder Colorado and the US Naval Observatory monitor Earth's rotation drift and recommend leap-second insertions to UTC to compensate for short-term irregularities.

Year 2026, our current year, is NOT a leap year: 2026 / 4 = 506.5, which is not an integer, so the year-divisible-by-4 test fails. February 2026 has 28 days. The next leap year is 2028 (2028 / 4 = 507), then 2032, 2036, 2040, and so on every four years until the next century-year exception in 2100 (which is divisible by 100 but not by 400, so 2100 will be a common year).

Different cultures use different calendars. The Islamic Hijri calendar has 354 or 355 days (purely lunar). The Hebrew calendar has 353 to 385 days depending on whether it is a common (12-month) or leap (13-month) year. The Hindu calendars vary by region. The Chinese lunisolar calendar inserts a leap month roughly every three years. The Bahá'í calendar adds 4 or 5 extra days (Ayyám-i-Há) at the end of the 18th month. But the international civilian standard is the Gregorian: 365 days in a common year, 366 days in a leap year.

The ISO 8601 standard, first published in 1988 and revised in 2004 and 2019 by the International Organization for Standardization, codifies the Gregorian calendar as the international reference. ISO 8601 also defines the ISO week-numbering system, which can produce a 53rd week in years where 1 January or 31 December falls on a Thursday. This is a separate count from the day-of-year count; an ISO 8601 year always has either 52 or 53 weeks (364 or 371 days at week boundaries).

Astronomically, Earth orbits the Sun in 365.256363 days (sidereal year) or 365.2422 days (mean tropical year, the time between vernal equinoxes). The Gregorian calendar is calibrated to the tropical year because that is what governs the seasons. The discrepancy of 0.014 days per year between sidereal and tropical years arises from the precession of Earth's axis - the slow conical wobble that moves the vernal equinox westward through the zodiac at about 50 arcseconds per year.

Citation footnotes: the Gregorian calendar leap-year rule appears in the 1582 papal bull Inter gravissimas and is codified in modern law via the UK Calendar (New Style) Act 1750 (adopted 1752) and the US Public Law 11-3-1751 (adopted 1752 for the British colonies). ISO 8601:2019 is the current international standard. The IANA Time Zone Database (TZDB) tracks all date / time rules. NIST and USNO publish ongoing tropical-year measurements based on VLBI (Very Long Baseline Interferometry) observations of distant quasars.