Gregorian Calendar - Description

Description

The Gregorian calendar is an arithmetical calendar. A Gregorian year consists of 365 or 366 days divided into twelve months:

An intercalary day in a leap year is known as a leap day. A leap day is the 29th day to February, which normally has 28 days. Since Roman times 24 February (bissextile) was counted as the leap day, but now 29 February is the leap day in most countries.

The main ongoing difference between the Gregorian and Julian calendars are the rules relating to the leap day. Unlike the Julian calendar, which inserts a leap day every 4 years, the Gregorian calendar omits 3 leap days every 400 years. This difference would have been more noticeable in modern memory were it not that the year 2000 was a leap year in both the Julian and Gregorian calendar systems.

Gregorian years are identified by consecutive year numbers. It is a solar calendar and counts days as the basic unit of time, grouping them into years of 365 or 366 days; and repeats completely every 146,097 days, which fill 400 years, and which also happens to be 20,871 seven-day weeks.

Of these 400 years, 303 common years have 365 days and 97 leap years have 366 days. This yields a calendar mean year of exactly 365+97/₄₀₀ days = 365.2425 days = 365 days, 5 hours, 49 minutes and 12 seconds. The same result is obtained by summing the fractional parts implied by the rule: 365 + 1⁄₄ − 1⁄₁₀₀ + 1⁄₄₀₀ = 365 + 0.25 − 0.01 + 0.0025 = 365.2425

Although the month length pattern is irregular, it can be represented by the arithmetic expression L = 30 + { MOD 2 }, where L is the month length in days and M is the month number 1 to 12. The expression is valid for eleven of the twelve months, but for M = 2 (February) must be adjusted by subtracting two, and then if it is a leap year adding one.

For a better representation, consider Zeller's Congruence.

A calendar date is fully specified by the year (numbered by some scheme beyond the scope of the calendar itself), the month (identified by name or number), and the day of the month (numbered sequentially starting at 1).

Although the calendar year runs from 1 January to 31 December, sometimes year numbers were based on a different starting point within the calendar. Confusingly, the term "Anno Domini" is not specific on this point, and actually refers to a family of year numbering systems with different starting points for the years. (See the section below for more on this issue.)