Chronological subjects

For example, February would have 29 days in a leap year instead of the usual 28. Seasons and astronomical events do not repeat at an exact number of full days, so a calendar which had the same number of days in each year would over time drift with respect to the event it was supposed to track. By occasionally inserting (or intercalating) an additional day or month into the year, the drift can be corrected. A year which is not a leap year is called a common year.

The term may implicitly refer to calendar millennia; periods tied numerically to a particular dating system, specifically ones that begin at the starting (initial reference) point of the calendar in question (typically the year 1) or in later years which are whole number multiples of a thousand years after it. The term can also refer to an interval of time beginning on any date. Frequently in the latter case (and sometimes also in the former) it may have religious or theological implications.

He obtained it from an Easter table attributed to Patriarch Cyril of Alexandria for the years 437–531. The latter was constructed around the year 440 by means of extrapolation from an Alexandrian Easter table constructed around the year 390 by Patriarch Theophilus of Alexandria. The great historical importance of Dionysius' Easter table is twofold:

ü From this Easter table Bede's Easter cycle would ultimately be developed by means of which all future Julian calendar dates of Easter Sunday were determined;

ü With his Easter table Dionysius introduced in passing the Christian era which would be developed into a full system for dating historical events by Bede two centuries later.

In the course of the third century Easter tables (tables with which dates of Easter Sunday can be found) came in use. In the beginning of the third century computists of some churches, among which the church of Rome and the one of Alexandria, had gone to calculate their own periodic sequences of dates of Paschal full moon, to be able to determine their own dates of Easter Sunday. To be able to develop those early Easter tables, owing to the then incalculability of the Hebrew calendar, one had been forced to substitute the dates of the fourteenth day of Nisan, preparation day of the Jewish Passover, for dates of the Paschal full moon adapted to either (e.g. in the case of the church of Rome) the Julian calendar or (e.g. in the case of the church of Alexandria) the Egyptian calendar. Obviously (periodic) sequences of dates of the Paschal full moon and (incalculable) sequences of dates of the fourteenth day of Nisan, initially as few different as possible, could in no case entirely tally with each other. During four centuries the sequences of dates of the Paschal full moon plied by the different churches could show great differences, which was the main cause of the fact that the Easter tables propagated then by the different churches all too often differed strongly and did not lead to the same dates being eligible for the celebration of Easter Sunday.

It occurs because leap years occur every 4 years and there are 7 possible days to start a leap year, making a 28 year sequence. This cycle also occurs in the Gregorian calendar, but it is interrupted by years such as 1700, 1800, 1900 and 2100, which are divisible by four but which are not leap years. This interruption has the effect of skipping 16 years of the solar cycle between February 28 and March 1. Because the Gregorian cycle of 400 years has exactly 146,097 days, i.e. exactly 20,871 weeks, one can say that the Gregorian so-called solar cycle lasts 400 years. Calendar years are usually marked by Dominical letters indicating the first Sunday in a new year, thus the term solar cycle can also refer to a repeating sequence of Dominical letters. Unless a year is not a leap year due to Gregorian exceptions, a sequence of calendars is reused every 28-years. The name solar cycle comes from Sunday, the traditional first day of the week.

It was used to identify the Roman year by a few Roman historians. Modern historians use it much more frequently than the Romans themselves did; the dominant method of identifying Roman years was to name the two consuls who held office that year. Before the advent of the modern critical edition of historical Roman works, AUC was indiscriminately added to them by earlier editors, making it appear more widely used than it actually was. The regnal year of the emperor was also used to identify years, especially in the Byzantine Empire after Justinian required its use in 537.

1. Ab Urbe condita (related with Anno Urbis conditae: AUC or a.u.c.) is Latin for "from the founding of the City (Rome)", traditionally set in 753 BC.

A leap year (or intercalary year) is a year containing one or more extra days (or, in case of lunisolar calendars, an extra month) in order to keep the calendar year synchronized with the astronomical or seasonal year.