def occurrences_between(t1, t2)
      return ((start_time < t1 || @start_time >= t2) ? [] : [start_time]) if @occurrence.nil?

      @occurrence.occurrences_between(t1, t2)
    end
def occurrences_between(t1, t2)
      return ((start_time < t1 || @start_time >= t2) ? [] : [start_time]) if @occurrence.nil?

      @occurrence.occurrences_between(t1, t2)
    end
occurrences in [t1, t2)