Hello everyone. I wanted to say thanks for this mod first of all. I am having one small error I cant seem to figure out.

I am using ZC v1.3.9h. It is a fresh install with no upgrades. The USPS auto fill is the only mod I have installed. The mod works as it should except for one thing. When I take the tracking info from USPS and post it in my orders screen and change it from pending to update. When I click the update button I get a http 500 error. If I click the back button it will update the order as it should. I checked my debug log and this is the error it is giving me.

[22-Dec-2011 20:01:20] PHP Fatal error: Call to undefined function date_diff() in /home1/outgrowc/public_html/admin/orders.php on line 146


This is what line 146 looks like

PHP Code:
$zc_max_days = date_diff($check_status->fields['date_purchased'], date('Y-m-d H:i:s', time())) + DOWNLOAD_MAX_DAYS; 
I was wondering if anyone has had this problem or did I mess something up on the install? This order.php file was renamed from the installation file as instructed.