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. 2  If you enter four digits, this is interpreted as the day and the month, and it will add the year of the work date. If the date you want to enter is in the range 01/01/1930 through 12/31/2029, you can enter the year with two digits; otherwise, enter the year with four digits. You can also enter a date as a weekday followed by a week number and, optionally, a year (for example, Mon25 or mon25 means Monday in week 25).