If you're using an earlier version of PHP, you'll need to enable the Date PHP4 module which provides some wrapper code to emulate those functions. The wrapper code is slower and less efficient than the native date functions, and has bugs that may never get totally fixed, so it is far better to use a server that has PHP 5.2 or higher enabled. Some distros, like Red Hat, use PHP 5.1 instead. See Installing PHP 5.2 on RedHat for ideas on how to update that.