how to set post time for add message

I don't think there's a parameter named "message.post_time" for that particular method. There is a "message.post_date" parameter listed in the documentation; however, I believe that particular parameter is reserved for internal Lithium use only. I'm going to follow up on getting the documentation updated to reflect this.

So unfortunately, I don't believe you're going to be able to set a specific time when posting a message. Is there a reason you want to set the time differently than the present time? Perhaps there's some other way to approach what you're trying to accomplish.