You can not use java.text.DateFormat within GWT/GXT applications. The class is not supported by the Google runtime environment. But there is a similar class com.google.gwt.i18n.client.DateTimeFormat which is useable:

DateTimeFormat dFormat = DateTimeFormat.getFormat("dd.MM.yyyy");
String date = dFormat.format(new Date());

DateTimeFormat tFormat = DateTimeFormat.getFormat("HH:mm:ss");
String time = tFormat.format(new Date());

This can be used within GridCellRenderer or ColumnConfig within a ColumnModel.

ColumnConfig config = new ColumnConfig("column", "Column", 100);
config.setDateTimeFormat(dFormat);