不妨定义一个类，实现 succ，<=> 方法，然后就可以用 Range 了：

class Ym
  include Comparable
  attr_accessor :year, :month

  def initialize(year, month)
    @year, @month = year, month
  end

  def succ
    yyy, mmm = @month == 12 ? [@year + 1, 1] : [@year, @month + 1]
    Ym.new(yyy, mmm)
  end

  def <=>(other)
    (@year * 12 + @month) <=> (other.year * 12 + other.month)
  end

  def to_s
    sprintf "%4d-%02d", @year, @month
  end
end

使用：

(Ym.new(2008, 9)..Ym.new(2009, 8)).each do |ym|
  puts ym
end

结果：

2008-09
2008-10
2008-11
2008-12
2009-01
2009-02
2009-03
2009-04
2009-05
2009-06
2009-07
2009-08