There are circumstances under which the date node does exist but is empty. In this case DialogDate throws a NullPointerException:
2004-12-05 18:32:07,386 [http-8180-Processor23] DEBUG jsp - Lack of configuration - not able to render dialog control
java.lang.NullPointerException
at info.magnolia.cms.gui.dialog.DialogDate.init(DialogDate.java:63)

Adding a test on null helps:
if (this.getWebsiteNode()!=null && this.getWebsiteNode().getNodeData(this.getName()).isExist()) {
Calendar valueCalendar=this.getWebsiteNode().getNodeData(this.getName()).getDate();
if ( valueCalendar != null )

{ Date valueDate = valueCalendar.getTime(); SimpleDateFormat sdf = new SimpleDateFormat(format); this.setValue(sdf.format(valueDate)); this.setWebsiteNode(null); //workaround so the value is taken... hm, pfusch }

}