The division of the year into months, weeks, and days and the method of ordering the years (past, present, and future).

From year one, an assumed date of the birth of Jesus, dates are calculated backwards (B.C., "before Christ" or B.C.E., "before common era") and forwards (A.D., Latin anno Domini, "in the year of the Lord", or C.E. "common era").

The meaning and origin as presented in this unit.

Calendars from Roman to modern times unit.

The period between one new Moon and the next which naturally averages 29.5 days, but the Western calendar uses, for convenience, a calendar month with a complete number of days, 30 or 31 (February has 28).

For adjustments, since there are slightly fewer than six extra hours a year left over, they are added to February 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.

A measure of the year based on twelve lunar months (the time it takes the moon to revolve around the earth), which lasts about 354.37 days.

A measure of the year based on earth's revolution around the sun, which takes 365.2422 days.

This is the calendar used in most of the world today.