...With a change Kier made on the back end (to increase the data storage to 28 days)...
Mike, I have gone from 10 to 30 days on my own vb forum, it caused massive confusion and it took 30days to propagate through.
After yesterday on xf.com when the change was made, threads I have already read are now marked as unread - even after I marked the entire xf.com site as read. Please don't worry about me particular situation.
(edit: you probably changed it more than once, again don't worry about that, not important)
However, I strongly recommend to pick a number of days, switch it now, and stick with that. Personally I would pick something like 90 days, or possibly longer. Then this issue will not be a problem for the casual visitors for xf, or any other xf powered site (assuming default settings are used). The people that have problems after 90 days, or the causal visitors reading threads over 90 days old which should be much more rare, can be helped on an individual basis.
If it is a server load issue, or speed issue, then admins could be told to change the default to something less.
From a technical point of view, could you say a few words about the timeout, and reason for it? I realize this is one of the
very few softwares (2?) that takes people to the last unread post, but
only considers posts displayed on the users screen as read. Other software seems to mark the entire thread as read even if only 1 page has been read, regardless of how long the thread is.