Once the Date API is installed, all functions in the API are available to be used anywhere by any module. If the Date Timezone module is installed, the system site timezone selector and the user timezone selectors are overwritten to allow the selection of timezone names instead of offsets. Proper timezone conversion requires knowledge of those timezone names, something that is not currently available in Drupal core, and the change in selectors makes it possible to track it.

In most cases, you should enable the Date Timezone module any time you use the Date API to be able to set the site and user timezone names. It is not enabled by default in case another module is setting timezone names in the database.

The API uses the PHP 5.2 date functions to create and manipulate dates, and contains an option module that will emulate those functions in earlier versions of PHP.

If you are using PHP 4 or PHP 5.0 or 5.1, native date handling won't work right. Install the Date_PHP4 module to enable wrapper functions so this code will work in old PHP versions.