MySQL date types are extremely flexible tools for storing date information. They are also extremely forgiving in the belief that it is up to the application, not the database, to validate date values. MySQL only checks that months range from 0 to 12 and dates range from 0 to 31. February 31, 2001, is therefore a legal MySQL date. More useful, however, is the fact that February 0, 2001, is a legal date. In other words, you can use 0 to signify dates in which you do not know a particular piece of the date.
Though MySQL is somewhat forgiving on the input format, you should actually attempt to format all date values in your applications in MySQL’s native format to avoid any confusion. MySQL always expects the year to be the left-most element of a date format. If you assign an illegal value in an SQL operation, MySQL will insert a zero for that value.
MySQL will also perform automatic conversion of date and time values to integer values when used in an integer context.