Converting a date string which is before 1970 into a timestamp in MySQL

I've adapted the DATEDIFF workaround to also include time not just days. I've wrapped it up into a stored function, but you can just extract the SELECT part out if you don't want to use functions.

DELIMITER |
CREATE FUNCTION SIGNED_UNIX_TIMESTAMP (d DATETIME)
RETURNS BIGINT
 DETERMINISTIC
  BEGIN
    DECLARE tz VARCHAR(100);
    DECLARE ts BIGINT;
    SET tz = @@time_zone;
    SET time_zone = '+00:00';
    SELECT DATEDIFF(d, FROM_UNIXTIME(0)) * 86400 +
    TIME_TO_SEC(
      TIMEDIFF(
        d,
        DATE_ADD(MAKEDATE(YEAR(d), DAYOFYEAR(d)), INTERVAL 0 HOUR)
      )
    ) INTO ts;
    SET time_zone = tz;
    return ts;
  END|
DELIMITER ;

-- SELECT UNIX_TIMESTAMP('1900-01-02 03:45:00');
-- will return 0
-- SELECT SIGNED_UNIX_TIMESTAMP('1900-01-02 03:45:00');
-- will return -2208888900

Aha! We've found a solution!

The SQL to do it:

SELECT DATEDIFF( STR_TO_DATE('04-07-1988','%d-%m-%Y'),FROM_UNIXTIME(0))*24*3600 -> 583977600
SELECT DATEDIFF( STR_TO_DATE('04-07-1968','%d-%m-%Y'),FROM_UNIXTIME(0))*24*3600 -> -47174400 

This could be useful for future reference.

You can test it here: http://www.onlineconversion.com/unix_time.htm

convert these date strings into a unix time stamp

Traditional Unix timestamps are an unsigned integer count of seconds since 1-Jan-1970 therefore can't represent any date before that.