A calendar is the division of the year into months, weeks, and days and the method
of ordering the years. From year one, an assumed date of the birth of Jesus, dates are
calculated backwards (BC 'before Christ' or BCE 'before common era') and forwards (AD,
Latin Anno Domini 'in the year of the lord' , or CE 'common era'). The lunar month (period
between one new moon and the next) naturally averages 29.5 days, but the Western calendar
uses for convenience a calendar month with a complete number of days, 30 or 31 (Feb has
28). For adjustments, since there are slightly fewer than six extra hours a year left
over, they are added to Feb as a 29th day every fourth year (leap year), century years
being excepted unless they are divisible by 400. For example, 1896 was a leap year. 1900
was not. 2000 is a leap year.