This Zen Cart article discusses the error message that reads ‘Fatal error: Cannot redeclare date_diff()’. As you may already know from the ‘fatal’ warning, this error message is pretty serious, and should be addressed as soon as possible.
Though serious, this is actually one of the simpler error messages you can get from your zen cart store. It basically means that the version of zen cart you are currently running is incompatible with the version of PHP being run by your server. In other words, your version of zen cart is too old to work with PHP. Zen cart v1.3.8a and older will not work with PHP v5.3.0 and newer. If you are getting this error message, there is only one thing you can do to fix it: you must immediately upgrade your store to the latest version of zen cart.
It should also be noted that, even when you are not seeing serious fatal error message like this one, if your zen cart version is older than v1.3.9, you should upgrade as soon as possible anyway. The latest version is always the best, as it includes the most recent bug fixes and other features, but there is still a more important reason to upgrade your store. This is that versions of zen cart older than 1.3.9 have serious security issues. If you are running this version of zen cart, you are running the risk of having your site broken in to, which can lead to all kinds of trouble, for you, and your customers. When an upgrade is performed correctly, it is quick and easy, with minimal downtime for your website, and there are tons of benefits that come from running your zen cart on the newest version.