Returns a date value from the year, month, and day values you enter. Salesforce displays an error on the detail page if the value of the DATE function in a formula field is an invalid date, such as February 29 in a non-leap year.
Use
DATE(year,month,day) and replace
year with a four-digit year, month with a two-digit
month, and day with a two-digit day.
Formula Example
DATE(2005, 01, 02)
creates a date field of January 2, 2005.