On my hosted instance, for this particular Case.
https://bhwh.fogbugz.com/default.asp?133#736
I tried several times adding time for today 1/3/2011 from 1:15 pm - 1:30 pm. Each and every time, when I clicked save, it saved the data as 1:15pm - 1:24pm. I changed the way I entered the time (was using 1.30 - and it changed it to 1:30pm, then I entered it directly as 1:30 pm), but it still changed it.
Even the elapsed time on the case showed 10 minutes.
Finally I had to adjust the hours to 1:00 pm - 1:15 pm to get it to work properly.