Leap Year Calender. A leap year is a year that has 366 days, instead of the usual 365 days. This is hard to calculate with, so a normal year has been given 365 days and a leap year 366.
To synchronize the calendar and tropical years, leap days are periodically added to the calendar, forming leap years. Any year evenly divisible by four is a leap year, except centesimal years (years ending in two zeros) which are considered common years and thus have the typical 365 days,.