date_default_timezone_set不适用于crond

When I call my php skript from the browser, it runs fine. When I call it through a cronjob, it returns this error:

Call to undefined function: date_default_timezone_set

Why? And how do I set the timezone properly?

if(function_exists('date_default_timezone_set')
{
   date_default_timezone_set('UTC');
}