The arguments of the UNIX_TIMESTAMP function can be omitted. If they are omitted, the function returns the interval between '1970-01-01 00:00:00' UTC and the current system date/time in seconds as INTEGER type. If the date argument is specified, the function returns the interval between '1970-01-01 00:00:00' UTC and the specified date/time in seconds.
0 is not allowed in the argument value corresponding to year, month, and day; however, if 0 is inputted in every argument value corresponding to date and time, 0 is returned as an exception.
UNIX_TIMESTAMP( [date] )
SELECT UNIX_TIMESTAMP('1970-01-02'), UNIX_TIMESTAMP();
unix_timestamp('1970-01-02') unix_timestamp()
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54000 1270196737
SELECT UNIX_TIMESTAMP ('0000-00-00 00:00:00');
unix_timestamp('0000-00-00 00:00:00')
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0