diff --git a/lib/encode.js b/lib/encode.js
index ee47929..d43dae2 100644
--- a/lib/encode.js
+++ b/lib/encode.js
@@ -5,7 +5,7 @@ var values = require('./values')
 
 var appleEpoch = Date.UTC(1904, 0, 1)
 var appleDate = function (value) {
-  if (util.isDate(value) === false) {
+  if (!(value instanceof Date)) {
     // value = new Date(value);
     throw new TypeError('Not a date: ' + value)
   }