YEAR[(2|4)]

A year in two-digit or four-digit format. The default is four-digit
format. In four-digit format, the allowable values are 1901 to 2155,
and 0000. In two-digit format, the allowable values are 70 to 69,
representing years from 1970 to 2069. MySQL displays YEAR values in
YYYY format, but allows you to assign values to YEAR columns using
either strings or numbers.

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

