Syntax:
UNIX_TIMESTAMP(), UNIX_TIMESTAMP(date)

If called with no argument, returns a Unix timestamp (seconds since
'1970-01-01 00:00:00' UTC). The return value is an integer if no
argument is given or the argument does not include a fractional seconds
part, or DECIMAL if an argument is given that includes a fractional
seconds part.

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. date
may be a DATE string, a DATETIME string, a TIMESTAMP, or a number in
the format YYMMDD or YYYYMMDD, optionally including a fractional
seconds part. 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/5.7/en/time-zone-support.html.

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

