Include custom field in Date Views Filter

Can you give me some more detail on how to include a custom DateTime field in the list of Dates that appear in the Views "Date" Filter?

I see a comment at Line 52 in date_api_fields.inc which seems to address the issue:

    // Custom fields should use the 'views_handler_field_date' handler and
    // return an array like array('sql_type' => DATE_ISO, 'tz_handling' => 'site');
    // when the hook is called with their table.field_name.

But I am not clear how or when to use the handler mentioned, or which hook is being referred to. Could you give a little more detail, and/or point to a simple example?

Many thanks,
Tony.