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.
How to get the week number from a date. To get the ISO week number (1-53), use idate('W', $timestamp) or strftime('%-V', $timestamp). To get the week number with a leading zero (01-53), use strftime('%V', $timestamp). $timestamp is a Unix timestamp, i.e. an integer value.