The default timezone is UTC time. You can reset your timezone here > Settings > General Select the closest city to you in the same timezone in the drop-down menu you see there and then click "save changes".
Please also make sure you have not unintentionally made any post a "sticky post". http://en.support.wordpress.com/posts/post-visibility/#sticky-posts
Blogs are essentially a reverse chronological order publishing tool wherein the most recent published post will display on top. You cannot change this order on a free hosted WordPress.com blog. It's designed to serve your visitors as they come to read the most recent posts and will not be inclined to trawl through hundreds of posts to locate it.