added calendar code

02.calendar/calendar.rb

@@ -0,0 +1,35 @@
+require 'date'
+require 'optparse'
+
+options = {}
+OptionParser.new do |opts|
+  opts.banner = "Usage: ruby calendar.rb [options]"
+
+  opts.on("-m", "--month MONTH", Integer, "Specify the month (1-12)") do |month|
+    options[:month] = month
+  end
+
+  opts.on("-y", "--year YEAR", Integer, "Specify the year") do |year|
+    options[:year] = year
+  end
+end.parse!
+
+current_date = Date.today
+year = options[:year] || current_date.year
+month = options[:month] || current_date.month
+
+date_range = Date.new(year, month, 1)..Date.new(year, month, -1)
+
+calendar_title = "#{month}月 #{year}"
+puts calendar_title.center(20)
+puts "日 月 火 水 木 金 土"
+
+initial_offset = date_range.first.wday
+
+print "   " * initial_offset
+
+date_range.each do |date|
+  print date.day.to_s.rjust(2) + " "
+
+  puts if date.wday == 6
+end