http://blog.jnito.com/entry/2013/05/01/082049

calendar.rb

require "time"

today = Time.now
#today = Time.parse('2013-04 07')

def to_calendar(time)
  target = time.dup

  def target.days
    (1..31).map do |day|
      Time.parse(strftime("%Y-%m-#{day}"))
    end.reject {|time| time.mon != mon }
  end

  def target.weeks
    ds = days.map{|time| time.strftime('%e')}
    ds.unshift days.first.strftime('%w').to_i.times.map{"  "}
    ds << (6 - days.last.strftime('%w').to_i).times.map{"  "}
    ds.flatten.each_slice(7)
  end

  def target.to_calendar
    width = 21
    <<TEXT
#{strftime('%B %Y').center(width)}
 Su Mo Tu We Th Fr Sa
#{weeks.map{|days| " " << days.join(" ")}.join("\n")}
TEXT
  end

  target.to_calendar
end

puts to_calendar(today)

puts
p today.respond_to?(:to_calendar) # => false