OK, I can replicate this with these settings. It didn't matter whether the time interval was 30 or 5. When I entered a single event on 05/14/2008 it would show on the Day View. To be honest I'm not sure why this is, I've done so many fixes around that set of code, but don't remember there ever being a bug with it not showing a single day event (it was a couple of months ago though).
In any case, I entered the same set of data against 4.0.0, and didn't get the problem. As I said before, I've done plenty of fixes in that area, so must have resolved this problem along the way.
Version 4.0.0 is very nearly into the release mechanism now, with the Release code being very, very similar to the RC3. I'm not sure how long it will take to get through the release process. However, I would strongly recommend upgrading to it when it is released, it has approximately 50 fixes in it.
Cheers
Roger