You're always welcome to post to old threads, so I wouldn't worry about the age of a story, but if it really digs at you the date a thread is created is stamped on the first post of that thread. Though if you aren't sure which came first, or in which order a story came in you might want to cross-reference the forums with a calender, your podcatcher, or the Pseudopod.org main page where all stories are listed in the order they are released.
I know this mean more work for you, and looking in more than one place to get an answer, but not everything can be as informative as
Wikipedia.