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.
What is a Date/Time? In Access, a date/time is actually a number, based on the number of days and fractions of a day since midnight December 30, 1899. Dates before that day are stored as negative numbers. Access can handle dates from January 1, 100 AD through Dec. 31, 9999 AD. Should be enough for most uses!