Hi Peppie,
As this functionality is built into the forum software, we don't always know all the specifics of the programming, however this is my understanding of these two function:
Today's Post - this lists all posts that have been updated in the last 24 hours.
New Posts - this should list threads/posts that have been updated since your last visit to the forum. There is a built-in time limit to this of 10 days or so.
This seems to work ok for me but not sure if others are having problems too.
For a lot more options and specific search features you can also use the
Forum Search at
http://www.flyingsolo.com.au/forums/search.php
Hope that helps!
Cheers
Peter