You asked for thread start date which is different from the date that is saved for a post.

Each are different things. Every new thread creates not only a thread record but a post record. The thread record records information relating to the thread as a whole and is cross-referenced with the actual posts through the threadid.

Now without looking at your actual data stored in the database, I can be sure where the problem is. I believe the date can be modified to reflect the date of the first post as opposed to the thread date but I will have to look into it.