Hi,
I know this is really old, but I found this forum when trying to figure out the same problem.
In the end the solution was really simple. This is what I did, hopefully it will help others as well:
>SELECT DATE_ADD(FROM_UNIXTIME(0), INTERVAL unix_timestamp SECOND)
where unix_timestamp is a positive or negative number.
e.g.
>SELECT DATE_ADD(FROM_UNIXTIME(0), INTERVAL -3600*24 SECOND)
will show '1969-12-31 12:00:00', or as in the above examples
>SELECT DATE_ADD(FROM_UNIXTIME(0), INTERVAL -13391999 SECOND)
will show '1969-07-30 12:00:01'.
