What is a leap year, and why do they happen? Everything to know about Leap Day
A leap year is a calendar year that has one extra day (or, in the case of a lunisolar calendar, one extra month) in comparison to a common year. It is sometimes referred to as an intercalary year or a bissextile year. To maintain the calendar year in line with the astronomical year or seasonal year, the 366th day (or 13th month) is added.[1] Calendars with a fixed number of days per year will inevitably drift over time with respect to the event that the year is designed to chronicle, such as seasons, since astronomical events and seasons do not recur in a complete number of days. A civilization’s dating system and the physical characteristics of some years might drift apart by “intercalating” (adding) a leap day or leap month.
Astronomical years have a duration of marginally less than 3651/4 days. In the ancient Julian calendar, February is extended to 29 days instead of the usual 28, and there are three common years of 365 days, followed by a leap year of 366 days. The most used civil calendar in the world, the Gregorian calendar, further corrects for the tiny inaccuracy in the Julian method. There are 366 days in a leap year as opposed to 365. Every year that is a multiple of four has an additional leap day, with the exception of those that are equally divisible by 100 but not by 400.
To prevent its calendar year from slipping through the seasons, the lunisolar Hebrew calendar adds the 13th lunar month, Adar Aleph, seven times every 19 years to the twelve lunar months of its common years. When necessary, a leap day is added to the Solar Hijri and Bahá’í calendars to guarantee that the next year starts on the March equinox.
The concept of a leap year is most likely derived from the Gregorian calendar, where a fixed date moves one day each week from one calendar year to the next. The day of the week for the 12 months (1 March to 28 February) following the leap day will move two days because of the extra day, resulting in a leap of one day per week. For example, a Friday in 2020 would fall on a Friday in 2021, while a Sunday in 2022 would fall on a Sunday in 2023, and a Monday in 2023 would fall on a Tuesday, and a Wednesday in 2024 would fall on a Wednesday.
Sometimes, due to variations in Earth’s rotation period, the length of a day can be adjusted by adding a leap second to Coordinated Universal Time.
Unlike leap days, however, leap seconds aren’t added on a regular basis because variations in the day’s length aren’t always predictable.
In computing, leap years can cause a problem called the leap year bug. This is when a year isn’t marked as a leap year, or when 29 February isn’t handled correctly in a date-accepting or date-mapping logic.