def duration(part = nil, klass = Duration)
          if [:years, :months, :weeks, :days, :hours, :minutes, :seconds].include? part
              klass.new(part => self)
          else
              klass.new(self)
          end
      end
def duration(part = nil, klass = Duration)
          if [:years, :months, :weeks, :days, :hours, :minutes, :seconds].include? part
              klass.new(part => self)
          else
              klass.new(self)
          end
      end
Create a Duration object using self where self could represent weeks, days,
 hours, minutes, and seconds.

 *Example*

     10.duration(:weeks)
     => #<Duration: 10 weeks>
     10.duration
     => #<Duration: 10 seconds>