String start=null,end=null;
       Date d1=null,d2=null;

start=(String)((CompletedReportsForm)form).getStart();
      end=(String)((CompletedReportsForm)form).getEnd();
       d1 =Date.valueOf(start);
      d2 =Date.valueOf(end);

JbpmQuery jbpmquery;
            jbpmquery=new JbpmQuery( "select s from s in class org.jbpm.model.execution.impl.ProcessInstanceImpl where s.end >=:d1 and s.end <=:d2 ");
            jbpmquery.addParameter("d1",d1);
            jbpmquery.addParameter("d2",d2);