String to timestamp in MySQL

UNIX_TIMESTAMP() does the trick:

SELECT UNIX_TIMESTAMP('2011-12-21 14:20:00');

However, the UNIX_TIMESTAMP() function only takes a standard MySQL formatted date. If you want to use the AM/PM notation, you will need to use STR_TO_DATE first like this:

SELECT UNIX_TIMESTAMP(
    STR_TO_DATE('2011-12-21 02:20pm', '%Y-%m-%d %h:%i%p')
);