Stored MySQL function to return a signed Unix timestamp compatible with pre Unix Epoch dates

signed_unix_timestamp.sql

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