Page "Calendar year" Paragraph 1

The Gregorian year begins on January 1 and ends on December 31 with a length of 365 days in an ordinary year and 366 days in a leap year giving an average length of 365. 2425 days.

Other formula-based calendars can have lengths which are further out of step with the solar cycle, for example, the Julian calendar has an average length of 365. 25 days, and the Hebrew calendar has an average length of 365. 2425