code: public Duration newDurationYearMonth(
              final boolean isPositive,
              final BigInteger year,
              final BigInteger month) {
  
          return newDuration(
                  isPositive,
                  year,
                  month,
                  null, // days
                  null, // hours
                  null, // minutes
                  null  // seconds
          );
      }
docs: <p>Create a <code>Duration</code> of type <code>xdt:yearMonthDuration</code> using the specified
  <code>year</code> and <code>month</code> as defined in
  <a href="http://www.w3.org/TR/xpath-datamodel#dt-yearMonthyDuration">
  XQuery 1.0 and XPath 2.0 Data Model, xdt:yearMonthDuration</a>.</p>
  
  <p>The XML Schema specification states that values can be of an arbitrary size.
  Implementations may chose not to or be incapable of supporting arbitrarily large and/or small values.
  An {@link UnsupportedOperationException} will be thrown with a message indicating implementation limits
  if implementation capacities are exceeded.</p>
  
  <p>A <code>null</code> value indicates that field is not set.</p>
  
  @param isPositive Set to <code>false</code> to create a negative duration. When the length
  of the duration is zero, this parameter will be ignored.
  @param year Year of <code>Duration</code>.
  @param month Month of <code>Duration</code>.
  
  @return New <code>Duration</code> created using the specified <code>year</code> and <code>month</code>.
  
  @throws IllegalArgumentException If any values would create an invalid <code>Duration</code>.
  @throws UnsupportedOperationException If implementation cannot support requested values.