# File lib/mail/parsers/rfc2822.rb, line 3346
    def _nt_year
      start_index = index
      if node_cache[:year].has_key?(index)
        cached = node_cache[:year][index]
        if cached
          cached = SyntaxNode.new(input, index...(index + 1)) if cached == true
          @index = cached.interval.end
        end
        return cached
      end

      i0 = index
      i1, s1 = index, []
      r2 = _nt_DIGIT
      s1 << r2
      if r2
        r3 = _nt_DIGIT
        s1 << r3
        if r3
          r4 = _nt_DIGIT
          s1 << r4
          if r4
            r5 = _nt_DIGIT
            s1 << r5
          end
        end
      end
      if s1.last
        r1 = instantiate_node(SyntaxNode,input, i1...index, s1)
        r1.extend(Year0)
      else
        @index = i1
        r1 = nil
      end
      if r1
        r0 = r1
      else
        r6 = _nt_obs_year
        if r6
          r0 = r6
        else
          @index = i0
          r0 = nil
        end
      end

      node_cache[:year][start_index] = r0

      r0
    end