// see this helper method below
Project project = CreateTestProjectWithCustomField();
        
ExtendedAttributeDefinition attr = project.getExtendedAttributes().get(0);
attr.setAlias("Days from finish to deadline");
attr.setFormula("[Deadline] - [Finish]");
        
java.util.Calendar cal = java.util.Calendar.getInstance();
cal.set(2015, 26, 3, 8,0,0);

Task task = project.getRootTask().getChildren().getById(1);
task.set(Tsk.DEADLINE, cal.getTime());

project.save("SaveFile.mpp", SaveFileFormat.MPP);