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The Gregorian Calendar is a solar calendar system used in much of the world. It consists of 7 day weeks and 12 months. One of those months, February, has an extra day in it on specific years (called "leap years") to align the calendar to the tropical year. This calendar is nearly the same as the Julian Calendar with only a few differences.
For years in AD (Anno Domini), also referred to as CE (Common Era), take the year. For years in BC (Before Christ), also referred to as BCE (Before Common Era), take the year minus 1.
If the value is (is divisible by 4) and (is not divisible by 100 or is divisible by 400), the year is a leap year, otherwise the year is not a leap year. Below is some pseudo code to show the process of determining if the year is a leap year:
function isLeapYear(integer year, boolean bc) boolean {
if (bc == true) {
year = year - 1
}
if (year % 4 == 0) {
if (year % 100 != 0 or year % 400 == 0) {
return true
}
}
return false
}
The Gregorian calendar, much like the Julian calendar, does not have a year 0. The year before year 1 AD (CE) is year 1 BC (BCE). There are also no negative number years, even though BC (BCE) years increment in number as you go backwards in time, much like negative numbers would.
Pope Gregory XIII issued this calendar reform to align the calendar more closely to the tropical year, as Easter was drifting further away from the Vernal Equinox (Spring/March Equinox). The reason for this drift was because of calculation rules created by the First Council of Nicea in 325 AD to calculate Easter soley using the Julian Calendar and not the Hebrew Calendar. Easter was set as the first Sunday after the first full moon that occurs on or after March 21st (an approximation of the Vernal Equinox).
Unfortunately, the Vernal Equinox is a tropical occurance and the Julian Calendar, while very close to a tropical year, was a bit off. The Julian Calendar had a leap year every 4th year, which sets the average year of the Julian Calendar to 365.25 days in length, as opposed to the average tropical year length of about 365.2421891 days. This means the Julian Calendar is slow compared to the average tropical calendar by 1 day about every 128 years. By 1545, when Poke Paul III was authorised to reform the calendar to realign the Vernal Equinox to the time of the First Council of Nicea, the Julian Calendar was slow by about 9.5 days.
Changing the leap year rules to have 97 leap years per 400 year period instead of 100 leap years per 400 year cycle set the average year of the Gregorian Calendar to 365.2425 days, which is slow by 1 compared to the average tropical calendar by 1 day about every 3217 years.
The first locations to implement the Gregorian Calendar were part of the Roman Empire in 1582. The change from the Julian Calendar to the Gregorian Calendar occurred in October, with the last day using the Julian Calendar being October 4th followed by the first day using the Gregorian Calendar being October 15th. October 5th through 14th were not observed that year for those switching calendar systems.
Other countries switched over time, with Great Britain and the British Empire switching in 1752, ending use of the Julian Calendar on September 2nd and starting use of the Gregorian Calendar on September 14th, skipping October 3rd through 13th.
Some countries switched to align with much of the rest of the world much later, but still use the Julian Calendar for religious celebrations. For instance, many Eastern Orthodox Christianity practitioners use the Julian Calendar for religious celebrations, as the Gregorian Calendar was a Catholic calendar reform.