The Julian calendar was created in 45 BC. Introduced by order of Julius Caesar, and the original intention was to make each fourth year a leap year, but this was not done correctly. Augustus ordered to omit a few leap years to solve the problem, and around 8 AD, leap years were observed every four years, and customs were consistent throughout modern times. Technically, a jump will have fewer birthday anniversaries than its age in years. This phenomenon is exploited when a person claims to be only a quarter of his real age by counting only his leap year anniversaries: in Gilbert and Sullivan`s comic opera The Pirates of Penzance of 1879, the apprentice pirate Frédéric discovers that he must serve the pirates until his 21st birthday (that is, when he will be 88 years old, since 1900 was not a leap year) and not before the age of 21. The following pseudocode determines whether a year is a leap year or an ordinary year in the Gregorian calendar (and in the proleptic Gregorian calendar before 1582). The year variable to be tested is the integer, which represents the number of the year in the Gregorian calendar. The observed and calculated versions of the Islamic calendar do not have regular leap days, although both have lunar months with 29 or 30 days, usually in alternating order. However, the Tabular Islamic Calendar, used by Islamic astronomers in the Middle Ages and still used by some Muslims, has a regular leap day added to the last month of the lunar year in 11 years of a 30-year cycle. [21] This extra day is at the end of last month, Dhu al-Hijjah, which is also the month of Hajj. [22] The Baháʼí calendar is a solar calendar consisting of 19 months of 19 days (361 days). The years begin in Naw-Rúz, at the spring equinox, on or around March 21.

Before the 19th month, a period of „Intercalary Days”, called Ayyam-i-Ha, is inserted. This period usually has 4 days, but an additional day is added as needed to ensure that the following year begins with the spring equinox. This is calculated years in advance and is known. The difference between the actual length of the year, 365.24237 days, and the assumed duration, 365.25 days, may not seem large, but over hundreds of years, the difference becomes obvious. The reason for this is that the seasons, which depend on the date of the tropical year, have become increasingly unbalanced with the date of the calendar. Pope Gregory XIII introduced the Gregorian calendar in 1582, which has been used ever since. To remedy this very slow drift, a new calendar was developed in the second half of the 16th century. Named after Pope Gregory XIII, the Gregorian calendar was published in 1582. In the modern calendar, the leap day falls on February 29. This was not always the case: when the Julian calendar was introduced, leap day was treated differently in two ways.

First of all, the leap day fell in February and not at the end. Second, the leap day was simply not counted, so a leap year still had 365 days. [4] Thanks to the leap year, our seasons will occur whenever we expect them to occur, and our calendar year will coincide with Earth`s sidereal year. From 8 AD, the Julian calendar received an additional day, which was added to February in years that are a multiple of 4 (although the numbering system of the AD years was not introduced until 525 AD). What does that mean? In the ancient Julian calendar, there were 100 leap years every 400 years. But in the Gregorian calendar, we only have 97 every 400 years. This is important because of the direction in which the points on the Earth`s axis control the seasons. When the southern hemisphere moves away from the sun, we experience our austral winter, while the northern hemisphere sees summer and vice versa. In the Hebrew lunisolar calendar, Adar Aleph, a 13th lunar month, is added seven times every 19 years to the twelve lunar months of his common years to prevent his calendar year from drifting through the seasons. In the Bahá`í calendar, a leap day is added as needed to ensure that the following year begins with the March equinox. Let us take the centuries of 1900 and 2000 as examples.

1900 is divisible by 100, but not by 400, so 1900 was not a leap year. For example, in the Gregorian calendar, each leap year has 366 days instead of 365 by extending February to 29 days instead of the usual 28. These extra days occur in each year which is an integer multiple of 4 (except for years that are divisible uniformly by 100, but not by 400). The 366-day leap year has 52 weeks and two days, so after a leap year, the year starts two days a week later. By having leap years every four years, we make sure that the months are consistent with the seasons. And if you were born on the leap day, February 29, that doesn`t mean you only celebrate one birthday every four years. Every three years, you can celebrate your birthday on March 1 and continue to age like the rest of us. One of the reasons for this rule is that Yom Kippur, the holiest day in the Hebrew calendar and the tenth day of the Hebrew year, should never again be adjacent to the weekly Sabbath (which is Saturday), that is, it should never fall on Friday or Sunday, so as not to have two adjacent Sabbath days.

However, Yom Kippur can still be Saturday. A second reason is that Hoshana Rabbah, the 21st day of the Hebrew year, will never be on Saturday. These rules for the holidays do not apply to the years from the creation to the liberation of the Hebrews from Egypt under Moses. At that time (cf. Exodus 13), the God of Abraham, Isaac, and Jacob gave the Hebrews their „law,” including holy days, feasts, and Sabbaths. The Hijri solar calendar is the modern Iranian calendar that is also used in Afghanistan. It is an observation schedule that begins with the spring equinox and adds a single day interspersed to the last month (Esfand) every four or five years; The first leap year occurs as the fifth year of the typical 33-year cycle, and the remaining leap years occur every four years for the remainder of the 33-year cycle. This system has less periodic deviation or jitter from its intermediate year than the Gregorian calendar and operates according to the simple rule that the spring equinox always falls within the 24-hour period that ends at noon on New Year`s Day. [23] The 33-year period is not entirely regular; From time to time, the 33-year cycle is interrupted by a 29-year cycle. [24] The additional rule that affects the centuries is an additional correction to compensate for the fact that an extra day every four years is a little too much correction (). This pattern causes the spring equinox to gradually shift its date between March 19 and 21, shifted once a leap year, and then abruptly shifted to non-leap centuries (see figure above). It may not seem like much, but over the centuries it has added up.

Until the 16th century, the spring equinox fell around March 11 instead of March 21. In 1582, Pope Gregory XIII adjusted the calendar by advancing the date by 11 days and introducing the exception to the rule for leap years. This new rule, according to which a centenary year is a leap year only if it is divisible by 400, is the only feature that distinguishes the Gregorian calendar from the Julian calendar. designated as February 29. A leap year occurs every four years to synchronize the calendar year with the solar year, or the time it takes to complete the Earth`s orbit around the sun, which is about 365 days and a quarter of a day. . . .