I do not think the accepted answer is a good universal solution!
This is because the UNIX_TIMESTAMP() function fails for DATEs before 1970-01-01 (and for dates in the far future using 32 bit integers). This may happen easily for the day of birth of many living people.
A better solution is:
SELECT TIMESTAMPDIFF(SECOND, '2010-11-29 13:13:55', '2010-11-29 13:16:55')
Which can be modified to return DAY YEAR MONTH HOUR and MINUTE too!