def seconds(part = nil)
          h = {:years => YEAR, :months => MONTH}
          if [:years, :months].include? part
              __send__(part) * h[part]
          else
              super(part)
          end
      end
def seconds(part = nil)
          h = {:years => YEAR, :months => MONTH}
          if [:years, :months].include? part
              __send__(part) * h[part]
          else
              super(part)
          end
      end
Derived from Duration#seconds, but supports `:years' and `:months' as well.