As stated elsewhere, there is no "date/timestamp" problem in this forum:
Time to untwist your knickers ladies! The reason you see a different 'last post' time stamp on the thread list when compared to the last post in the thread is because there is a poll in the threads you are fretting about.
If a thread has a poll attached then every time someone votes in the poll the thread 'last post' time is updated. If this didn't happen and no one actually made a new post to the thread with the poll then that thread would slip down the list until it wasn't on the first page as other threads were updated with new posts.
It's not rocket science and nothing is broken. Just one of the ways threads with polls stay 'active' if there is a lot of voting but no new posts.