After successfully installing Zen Cart on my Fedora 12 Linux Box, I immediately got a blank screen when I tried either the main page or the admin page. After using the information found in the tutorial, "I am getting a blank page (or blank part of a page)", I realized that Zen Cart was defining the function
date_diff in both includes/functions/functions_general.php and in admin/includes/functions/general.php.

I fixed this problem by enclosing both definitions in a
if(!function_defined("date_diff")){
function date_diff($date1, $date2) {

//$date1 today, or any other day

//$date2 date to check against



$d1 = explode("-", $date1);

$y1 = $d1[0];

$m1 = $d1[1];

$d1 = $d1[2];



$d2 = explode("-", $date2);

$y2 = $d2[0];

$m2 = $d2[1];

$d2 = $d2[2];



$date1_set = mktime(0,0,0, $m1, $d1, $y1);

$date2_set = mktime(0,0,0, $m2, $d2, $y2);



return(round(($date2_set-$date1_set)/(60*60*24)));

}

}