Calendar

===Gregorian===
The [[Gregorian calendar]] is the ''[[de facto]]'' international standard and is used almost everywhere in the world for civil purposes. The widely-used solar aspect is a cycle of leap days in a 400-year cycle designed to keep the duration of the year aligned with the [[solar year]].{{sfn | Blackburn | Holford-Strevens | 2003 | pp = 682&ndash;683}} There is a lunar aspect which approximates the position of the moon during the year, and is used in the [[Computus | calculation of the date of Easter]].{{sfn | Blackburn | Holford-Strevens | 2003 | pp = 817&ndash;820}} Each Gregorian year has either 365 or 366 days (the leap day being inserted as 29 February), amounting to an average Gregorian year of 365.2425 days (compared to a solar year of 365.2422 days).{{sfn | Dershowitz | Reingold | 2008 | pp = 47, 187}}

The calendar was introduced in 1582 as a refinement to the [[Julian calendar]], which had been in use throughout the European Middle Ages, amounting to a 0.002% correction in the length of the year.{{sfn | Blackburn | Holford-Strevens | 2003 | pp = 682&ndash;683}} During the Early Modern period, [[Adoption of the Gregorian calendar|its adoption was mostly limited]] to [[Roman Catholic]] nations, but by the 19th century it had become widely adopted for the sake of convenience in international trade. The last European country to adopt it was Greece, in 1923.{{sfn | Blackburn | Holford-Strevens | 2003 | pp = 682&ndash;689}}

The [[calendar epoch]] used by the Gregorian calendar is inherited from the medieval convention established by [[Dionysius Exiguus]] and associated with the Julian calendar. The year number is variously given as AD (for ''[[Anno Domini]]'') or CE (for ''[[Common Era]]'' or ''Christian Era'').{{sfn | Blackburn | Holford-Strevens | 2003 | loc= Chapter: "Christian Chronology"}}