Syntax:
TIMEDIFF(expr1,expr2)

TIMEDIFF() returns expr1 − expr2 expressed as a time value. expr1 and
expr2 are time or date-and-time expressions, but both must be of the
same type.

The result returned by TIMEDIFF() is limited to the range allowed for
TIME values. Alternatively, you can use either of the functions
TIMESTAMPDIFF() and UNIX_TIMESTAMP(), both of which return integers.

URL: http://dev.mysql.com/doc/refman/5.7/en/date-and-time-functions.html

