Convert at only if provided

www/local-notification-util.js

@@ -154,11 +154,13 @@ exports.convertProperties = function (options) {
         }
     }
 
+    if (options.at) {
         if (typeof options.at == 'object') {
             options.at = options.at.getTime();
         }
 
         options.at = Math.round(options.at/1000);
+    }
 
     if (typeof options.data == 'object') {
         options.data = JSON.stringify(options.data);