gistfile1.rb

  # return the date at which the week with the given number starts
  # (ISO-8601)
  #
  #  Date.beginning_of_numbered_week(2003, 52)   # => 2003-12-22
  #  Date.beginning_of_numbered_week("2003W52")  # => 2003-12-22
  #
  def self.beginning_of_numbered_week(year, week = nil)
    if week
      year, week = year.to_i, week.to_i
    else
      year, week = year.scan(/(\d{4})-?W(\d{1,2})/)[0].map(&:to_i)
    end

    raise ArgumentError unless year and week

    first_week_of_year = Date.new(year.to_i, 1, 4).monday
    start_day = first_week_of_year + ((week-1) * 7).days

    raise ArgumentError if start_day.year > year
    return start_day
  end