don't check for Date instances, just required methods

lib/index.js

@@ -57,7 +57,7 @@
   // locale is an object with the same structure as DefaultLocale
   function _strftime(fmt, d, locale, _useUTC) {
     // d and locale are optional so check if d is really the locale
-    if (d && !(d instanceof Date)) {
+    if (d && !quacksLikeDate(d)) {
       locale = d;
       d = undefined;
     }
@@ -135,6 +135,19 @@
     });
   }
 
+  RequiredDateMethods = ['getTime', 'getTimezoneOffset', 'getDay', 'getDate', 'getMonth', 'getFullYear', 'getYear', 'getHours', 'getMinutes', 'getSeconds'];
+  function quacksLikeDate(x) {
+    var i = 0
+      , n = RequiredDateMethods.length
+      ;
+    for (i = 0; i < n; ++i) {
+      if (typeof x[RequiredDateMethods[i]] != 'function') {
+        return false;
+      }
+    }
+    return true;
+  }
+
   // Default padding is '0' and default length is 2, both are optional.
   function pad(n, padding, length) {
     // pad(n, <length>)