Calendar year ("leap days" or "intercalary days"). The Gregorian year, which is in use in most of the world, begins on January 1 and ends on December 31. It has a length of 365 days in an ordinary year, with 8,760 hours, 525,600 minutes, or 31,536,000 seconds; but 366 days in a leap year, with 8,784 hours, 527,040 minutes, or 31,622,400 seconds. With 97 leap years every 400 years, the year has 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
Calendar reform The Common Civil Calendar and Time calendar has months of 30 and 31 days, but inserts a leap week in the middle of the year, when needed, whereas its successor, the Hanke-Henry Permanent Calendar, moves the extra week to the end of the year. In the World Season Calendar, months are discarded altogether; instead, the year is divided into four seasons of 13 weeks each. An extra day (two days during leap year) is added to the calendar that is not assigned a day of the week in order to perennialize the calendar. The same calendar of 91 days is