convert original time to Date object

lib/instance.js

@@ -345,6 +345,9 @@ Instance.prototype.set = function(key, value, options) { // testhint options:non
             if (!(value instanceof Date)) {
             if (!(value instanceof Date)) {
               value = new Date(value);
               value = new Date(value);
             }
             }
+            if (!(originalValue instanceof Date)) {
+              originalValue = new Date(originalValue);
+            }
             if (originalValue && value.getTime() === originalValue.getTime()) {
             if (originalValue && value.getTime() === originalValue.getTime()) {
               return;
               return;
             }
             }