code: public static Date truncate(final Date date, final int field) {
          validateDateNotNull(date);
          final Calendar gval = Calendar.getInstance();
          gval.setTime(date);
          modify(gval, field, ModifyType.TRUNCATE);
          return gval.getTime();
      }
docs: <p>Truncates a date, leaving the field specified as the most
  significant field.</p>
  
  <p>For example, if you had the date-time of 28 Mar 2002
  13:45:01.231, if you passed with HOUR, it would return 28 Mar
  2002 13:00:00.000.  If this was passed with MONTH, it would
  return 1 Mar 2002 0:00:00.000.</p>
  
  @param date  the date to work with, not null
  @param field  the field from {@code Calendar} or <code>SEMI_MONTH</code>
  @return the different truncated date, not null
  @throws IllegalArgumentException if the date is <code>null</code>
  @throws ArithmeticException if the year is over 280 million