February 29 is a date that usually occurs every four years, and is called leap day. This day is added to the calendar in leap years as a corrective measure, because the Earth does not orbit the sun in precisely 365 days. The Gregorian calendar is a modification of the Julian calendar first used by the Romans.
For example, in the formula DATE( 2008, 5, -15), the DATE function returns a datetime value equivalent to April 15th of 2008, because 15 days are subtracted from the beginning of May 2008 yielding a value of April 2008.
=DAYS(3/15/11,2/1/11) Finds the number of days between the end date (3/15/11) and end date (2/1/11). When you enter a date directly in the function, you need to enclose it in quotation marks. Result is 42. Finds the number of days between the end date in A2 and the start date in A3 (364).