code: public static Calendar truncate(final Calendar date, final int field) {
          if (date == null) {
              throw new IllegalArgumentException("The date must not be null");
          }
          final Calendar truncated = (Calendar) date.clone();
          modify(truncated, field, ModifyType.TRUNCATE);
          return truncated;
      }
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