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.