Mary Anne pointed out that the method I was using to generate "Previous" and "Next" links resulted in the browser not correctly indicating which entries you've viewed before. I rely heavily in my own Web-browsing on the browser coloring links I've already visited differently from links I haven't, so I figured out a way to fix the problem.
So I think the Next and Previous buttons should still behave correctly, and links on the today's-entries page should be colored correctly from now on. (For those uncertain about navigation, the URL to use for this journal is <http://www.kith.org/logos/journal/>—that URL will always show the titles of the current day's entries.) As a side benefit, the "Next" link now correctly goes away when there's no next entry, and the "Previous" link correctly goes away when there's no previous entry (which is only true for the first entry). Wonder if I should add "First" and "Last" links to the navigation links?
Anyway, let me know if you encounter any strangenesses in navigation, particular in the way Next and Previous now work.
(Note that all this applies only if you're viewing the entries one at a time. If you view all the entries for one day at once, I don't think there's any way I can make the browser color links to the individual entries as having been visited; the individual-entry page is a separate page from the all-entries-for-one-day page.)