code: public static Calendar startOfDay(Calendar origin) {
          Calendar cal = (Calendar) origin.clone();
          cal.set(Calendar.MILLISECOND, 0);
          cal.set(Calendar.SECOND, 0);
          cal.set(Calendar.MINUTE, 0);
          cal.set(Calendar.HOUR_OF_DAY, 0);
          return cal;
      }
docs: Calculate the start-of-day point of a supplied {@link Calendar}.
  
  <p>
  The returned {@link Calendar} has its fields unsynced. Note: if {@code origin} has
  unsynced field, it may cause side-effect.
  </p>
  
  @param origin
  @return