# File lib/ice_cube/occurrence.rb, line 88
    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