February 29, also known as leap day or leap year day, is a date added to most years that are divisible by 4, such as 2008, 2012, 2016, 2020, and 2024. A leap day is added in various solar calendars (calendars based on the Earth's rotation around the Sun), including the Gregorian calendar standard in most the world. Lunisolar calendars (calendars based on the rotation of the Moon) instead add a leap or intercalary month.In the Gregorian calendar, years that are divisible by 100, but not by 400, do not contain a leap day. Thus, 1700, 1800, and 1900 did not contain a leap day, 2100, 2200, and 2300 will not contain a leap day, while 1600 and 2000 did, and 2400 will. Years containing a leap day are called leap years. February 29 is the 60th day of the Gregorian calendar in such a year, with 306 days remaining until the end of the year. In the Chinese calendar, this day will only occur in years of the monkey, dragon, and rat. A leap day is observed because a complete revolution around the Sun takes approximately 6 hours longer than 365 days (8,760 hours). It compensates for this lag, realigning the calendar with the Earth's position in the Solar System; otherwise, seasons would occur in a different time than intended in the calendar year. The Gregorian calendar has the century rules to the leap year to ensure that Easter occurs near the vernal equinox, or spring in the northern hemisphere.