Syntax:
TIMESTAMPDIFF(unit,datetime_expr1,datetime_expr2)

Returns the integer difference between the date or datetime expressions
datetime_expr1 and datetime_expr2. The unit for the result is given by
the unit argument. The legal values for unit are the same as those
listed in the description of the TIMESTAMPADD() function.

URL: http://dev.mysql.com/doc/refman/5.0/en/date-and-time-functions.html

