Ah, I think I've got why you are having troubles with using this code in your web part. These methods are supposed to be used in ASCX Transformations, not in the web parts. But you can try the following method instead:
<%@ Import Namespace="CMS.DocumentEngine.Web.UI" %>
...
<%# TransformationHelper.HelperObject.GetCustomDateTime(DateTime.Now, "CentralAmerica") %>