Syntax:
UNIX_TIMESTAMP(), UNIX_TIMESTAMP(date)

If called with no argument, returns a Unix timestamp (seconds since
'1970-01-01 00:00:00' UTC) 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' UTC. The date argument may be a
DATE, DATETIME, or TIMESTAMP string, or a number in YYMMDD,
YYMMDDHHMMSS, YYYYMMDD, or YYYYMMDDHHMMSS format. The server interprets
date as a value in the current time zone and converts it to an internal
value in UTC. Clients can set their time zone as described in
http://dev.mysql.com/doc/refman/5.5/en/time-zone-support.html.

URL: http://dev.mysql.com/doc/refman/5.5/en/date-and-time-functions.html

