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.
In a date field you can enter two, four, six, or eight digits: 1  If you enter only two digits, this is interpreted as the day, and it will add the month and the year of the work date.  If you enter four digits, this is interpreted as the day and the month, and it will add the year of the work date.