Ruby DateTime iso8601 for week dates

datetime_ext.rb

class DateTime
  # Regex to match Week date from ISO8601 represented as YYYY-Www
  # For more information about Week date see: https://en.wikipedia.org/wiki/ISO_8601#Week_dates
  #
  SHORT_FORMAT_WEEK_DATE_REGEX = /\A[0-9]{4}-W([0-4]\d|5[0-2])\z/.freeze

  class << self
    alias_method :ruby_iso8601, :iso8601

    # Since Ruby supports the longer week date (YYYY-Www-D) and 2017-W29 is the
    # same as write 2017-W29-1 we just add the week day when the given "string"
    # parameter is the shorter version of a Week date.
    #
    def iso8601(string = '-4712-01-01T00:00:00+00:00', start = Date::ITALY)
      date = string.to_s.dup
      
      if SHORT_FORMAT_WEEK_DATE_REGEX.match?(date)
        date << '-1'
      end
      
      ruby_iso8601(date, start)
    end
  end
end