When I used PHP’s mktime() on FreeBSD for dates prior to Jan 1, 1970, it was fine. Used the same code on RHEL 3, it gave me an incorrect value (-14462).
This is due to glibc library in some OS not allowing negative timestamps: http://bugs.php.net/bug.php?id=20048.
Read http://phplens.com/phpeverywhere/adodb_date_library for a solution to this issue.