[vb3.0] Marking posts in a thread as "old"... properly. HOW?

[dogbomb]

See in the top left of this posts, there is an icon. When it is a new post it is:


And an older post is:


Well, when you have a new post... and leave the thread and come back before the cookie has timed it out. It remains NEW!

Is there any way to mark all posts within a thread as OLD, when the thread has been looked at?

Thanks in advance for any help.