EDIT: I just realized, this only converts to a timestamp, not to a formatted date. DATE_FORMAT(), as mentioned above, is what you need. Ignore my post.

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From http://www.mysqlfreaks.com/statements/68.php.

UNIX_TIMESTAMP:

Statement Info:
If called with no argument, returns a Unix timestamp (seconds since '1970-01-01 00:00:00' GMT) as an unsigned integer. If UNIX_TIMESTAMP() is called with a date argument, it returns the value of the argument as seconds since '1970-01-01 00:00:00' GMT. date may be a DATE string, a DATETIME string, a TIMESTAMP, or a number in the format YYMMDD or YYYYMMDD in local time.
Examples:
SELECT UNIX_TIMESTAMP();
-> 882226357

SELECT UNIX_TIMESTAMP('1997-10-04 22:23:00');
-> 875996580

So, you should be able to do something like UNIX_TIMESTAMP(fieldname) and have it convert for you. Why go through all the bother of doing it yourself?