code: function headerReduce(headers, format) {
      var minmtime = new Date('Sun, 23 Feb 2014 18:00:00 UTC');
      var composed = {};
  
      composed['Cache-Control'] = 'max-age=3600';
  
      switch (format) {
      case 'vector.pbf':
          composed['Content-Type'] = 'application/x-protobuf';
          composed['Content-Encoding'] = 'deflate';
          break;
      case 'jpeg':
          composed['Content-Type'] = 'image/jpeg';
          break;
      case 'png':
          composed['Content-Type'] = 'image/png';
          break;
      }
  
      var times = headers.reduce(function(memo, h) {
          if (!h) return memo;
          for (var k in h) if (k.toLowerCase() === 'last-modified') {
              memo.push(new Date(h[k]));
              return memo;
          }
          return memo;
      }, []);
      if (!times.length) {
          times.push(new Date());
      } else {
          times.push(minmtime);
      }
      composed['Last-Modified'] = (new Date(Math.max.apply(Math, times))).toUTCString();
  
      var etag = headers.reduce(function(memo, h) {
          if (!h) return memo;
          for (var k in h) if (k.toLowerCase() === 'etag') {
              memo.push(h[k]);
              return memo;
          }
          return memo;
      }, []);
      if (!etag.length) {
          composed['ETag'] = '"' + crypto.createHash('md5').update(composed['Last-Modified']).digest('hex') + '"';
      } else {
          composed['ETag'] = etag.length === 1 ? etag[0] : '"' + crypto.createHash('md5').update(etag.join(',')).digest('hex') + '"';
      }
  
      return composed;
  }
docs: Calculate TTL from newest (max mtime) layer.