code: public static final String roll(String isoString, int field, boolean up)
              throws ParseException {
  
          return roll(isoString, DATETIME_PATTERN, field, up);
      }
docs: Time Field Rolling function. Rolls (up/down) a single unit of time on the
  given time field.
  
  @param isoString
  @param field
  the time field.
  @param up
  Indicates if rolling up or rolling down the field value.
  @exception ParseException
  if an unknown field value is given.