date_string_or_error attempt, not working

date_string_or_error.ex

  def modified_date_or_datetime(string, modifier) do
    date_match = Date.from_iso8601(string)
    iso_match  = Timex.parse(string, "{ISO:Extended}")
    isoz_match = Timex.parse(string, "{ISO:Extended:Z}")

    case {:ok, x }  do
      ^date_match -> "#{string}#{modifier}"
      ^iso_match -> string
      ^isoz_match -> string
      _ -> raise "Invalid ISO8601 date/datetime format provided: #{string}"
    end
  end

initial_attempt.ex

  def modified_date_or_datetime(string, modifier) do
    cond do
      {:ok, _parsed_date} = Date.from_iso8601(string) -> "#{string}#{modifier}"
      {:ok, _parsed_datetime} = Timex.parse(string, "{ISO:Extended}") -> string
      {:ok, _parsed_datetime} = Timex.parse(string, "{ISO:Extended:Z}") -> string
      true -> raise "Invalid ISO8601 date/datetime format provided: #{string}"
    end
  end