   UNIX_TIMESTAMP()
   UNIX_TIMESTAMP(date)
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.
