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
Thirty Days Hath September Meton in the 5thcentury. Rather than adopt a new system like the Egyptian calendar, which had 12 months of 30 days each and a set, annual intercalary month of 5 days, Caesar aimed for his 46 reform to maintain as much continuity as possible with the old calendar. Ultimately, Mercedonius was removed, the four existing 31-day months were maintained, February was left unchanged apart from leap years, and the needful additional ten days of the year were added to the 29-day months to make them either 30 or 31 days long. By the Renaissance, the irregularity of the resulting system