diff --git a/lib/util.js b/lib/util.js
index 5a3ac76c..31ca9bb7 100644
--- a/lib/util.js
+++ b/lib/util.js
@@ -409,7 +409,7 @@ exports.convert = function (object, type) {
 
     case 'number':
     case 'Number':
-      if (!isNaN(Date.parse(object))) {
+      if (exports.isString(object) && !isNaN(Date.parse(object))) {
         return moment(object).valueOf();
       } else {
         return Number(object.valueOf());