diff --git a/lib/date.js b/lib/date.js
index 13bcc15..d290698 100644
--- a/lib/date.js
+++ b/lib/date.js
@@ -43,7 +43,6 @@ if (!Date.prototype.getFullWeek) {
 if (!Date.prototype.format) {
   Object.defineProperty(Date.prototype, 'format', {
     value: function(str) {
-      str = str || 'Y-m-d H:i:s'
       var dt = {
         fullyear: this.getFullYear(),
         year: this.getYear(),
@@ -58,6 +57,7 @@ if (!Date.prototype.format) {
       }
       var reg = null
 
+      str = str || 'Y-m-d H:i:s'
       dt.g = dt.hours > 12 ? dt.hours - 12 : dt.hours
 
       reg = {