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.