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 (the period between one new moon and the next) naturally averages 29.5 days. For convenience the Western calendar divides a calendar month into full days, 30 or 31 (Feb has 28).

For adjustments, since there are slightly fewer than six extra hours a year left over, February has an extra day  every fourth year (leap year). To determine if a year is a leap year divide it by 400. If the year is divisible by 400 it is a leap year.

 

For example, 1896 was a leap year. 1900 was not. 2000 is a leap year.