Syntax:
DATEDIFF(expr1,expr2)

DATEDIFF() returns expr1 - expr2 expressed as a value in days from one
date to the other. expr1 and expr2 are date or date-and-time
expressions. Only the date parts of the values are used in the
calculation.

URL: http://dev.mysql.com/doc/refman/5.1/en/date-and-time-functions.html

