Which language are you making API requests? I'm digging deeper and looks like, Timestamp should be current request time, so you must generate timestamp on request. Have you looked at their REST API Demos? Here is url https://github.com/huobiapi/REST-API-demos
I found the problem - the time should be strictly Greenwich (UTC-0) time without any offsets.
Thank you for your help!
I sends API in PHP and C#. If you need something - I'm always ready to help.