Fix time synchronization with server

- avoid that client time being ahead of server time

lib/request.js

@@ -43,7 +43,7 @@ module.exports = class Request {
     const result = await this.get('/v2/public/time');
     const end = Date.now();
 
-    return Math.ceil((result.time_now * 1000) - start + ((end - start) / 2));
+    return Math.ceil((result.time_now * 1000) - end + ((end - start) / 2));
   }
 
   async _call(method, endpoint, params) {