diff --git a/lib/util.js b/lib/util.js
index 1a041906db3..f47e39fddda 100644
--- a/lib/util.js
+++ b/lib/util.js
@@ -332,11 +332,8 @@ function isRegExp(re) {
 
 
 function isDate(d) {
-  if (d instanceof Date) return true;
-  if (typeof d !== 'object') return false;
-  var properties = Date.prototype && Object.getOwnPropertyNames(Date.prototype);
-  var proto = d.__proto__ && Object.getOwnPropertyNames(d.__proto__);
-  return JSON.stringify(proto) === JSON.stringify(properties);
+  return d instanceof Date ||
+    (typeof d === 'object' && Object.prototype.toString.call(d) === '[object Date]');
 }