AFAIK it is an issue of the forums module, which doesn't properly convert system time (stored in the database) to user time.

It would be a big step forward, if modules would store datetime values in UTC and adjust to user time upon display, as this would allow moving the server to a different time zone without the need of data corrections.

What makes sense for every user is local time to the user. For authenticated users they can change it to something else. The whole point of a timestamp is to know how old a post is if it's a same day post. In some forum software I have seen, if the new post is posted few minutes ago, the time stamp will look something like '5 minutes ago'. No timezone involved. This makes sense. StackExchange/StackOverflow does that. 

The proper timestamp calculation is simple to do. Save the time  when the post is created in SQL Server using UTC time. Get the user's timezone through javascript and then convert the UTC time to that time zone when displaying the posts. Although I guess there needs to be a second request done asynchronously to send the unauthenticated user's time zone to the server? Can't determine it from the first request.