宿題スレに書き込んだのが残ってた
calコマンドのように表示するように作ったのは確かなんだが、
ぶっちゃけ表示部で何やってんだかわからん

#!/usr/bin/ruby
require 'date'
### Usage: ./cal.rb [YYYY MM]
### ./cal.rb 2007 3 #=> Mar 2007 calender
### ./cal.rb 3 2007 #=> Mar 2007 calender
### ./cal.rb #=> this month's calender
if ARGV.empty? then
t = Date.today
else
argv = ARGV[0,2].map{|s| s.to_i}
t = Date.new(argv.max{|a,b| a <=> b}, argv.min{|a,b| a <=> b}, 1)
end
(d_one, d_last) = Date.new(t.year, t.month, 1), Date.new(t.year, t.month, -1)
cal = "#{Date::MONTHNAMES[d_one.month]} #{d_one.year}".center(3*7) << "\n"
cal << Date::ABBR_DAYNAMES.inject(''){|days,name| days << name.chop.rjust(2) << ' '} << "\n"
(d_one-d_one.wday).upto(d_last+6-d_last.wday) do |d|
(cal << ' ';next) unless d.month == d_one.month
cal << d.day.to_s.rjust(2) << ' ' << "\n"*(d.wday / 6)
end
puts cal