def to_s(format=nil)
      if format && to_time.public_method(:to_s).arity != 0
        t0, t1 = start_time.to_s(format), end_time.to_s(format)
      else
        t0, t1 = start_time.to_s, end_time.to_s
      end
      duration > 0 ? "#{t0} - #{t1}" : t0
    end
def to_s(format=nil)
      if format && to_time.public_method(:to_s).arity != 0
        t0, t1 = start_time.to_s(format), end_time.to_s(format)
      else
        t0, t1 = start_time.to_s, end_time.to_s
      end
      duration > 0 ? "#{t0} - #{t1}" : t0
    end
Shows both the start and end time if there is a duration.
 Optional format argument (e.g. :long, :short) supports Rails
 time formats and is only used when ActiveSupport is available.