My system is upgraded from v1.3.8a to v1.3.9f on a php v5.3.2 server, and I keep receveing such error message:

"PHP Fatal error: Cannot redeclare date_diff() in /shop/includes/functions/functions_general.php on line 1479"


Line 1479 is:
" $skipPreg = (defined('OVERRIDE_REPLACE_ACCENTS_WITH_HTMLENTITIES') && OVERRIDE_REPLACE_ACCENTS_WITH_HTMLENTITIES == 'TRUE') ? TRUE : FALSE;"

I search the whole file, and in line 1454:
"function zen_date_diff($date1, $date2) {"


Did I still miss something while upgrading?