code: private Calendar cleanHistCalendar(Calendar cal) {
          cal.set(Calendar.MILLISECOND, 0);
          cal.set(Calendar.SECOND, 0);
          cal.set(Calendar.MINUTE, 0);
          cal.set(Calendar.HOUR, 0);
          return cal;
      }
docs: Put everything smaller than days at 0
  @param cal calendar to be cleaned