def strftime(fmt)
          h = {'y' => @years, 'M' => @months}
          super(fmt.gsub(/%?%(y|M)/) { |match| match.size == 3 ? match : h[match[1..1]] })
      end
def strftime(fmt)
          h = {'y' => @years, 'M' => @months}
          super(fmt.gsub(/%?%(y|M)/) { |match| match.size == 3 ? match : h[match[1..1]] })
      end
Similar to Duration.new except that BigDuration.new supports `:years' and
 `:months' and will also handle years and months correctly when breaking down
 the seconds.

 BigDuration variant of Duration#strftime.

 *Identifiers: BigDuration*

     %y -- Number of years
     %m -- Number of months