Observed: When I filtered tournament results by a single day for 2010-03-31, even though I had data for that date in my tournament results, I didn't see it. When I filtered for 2010-04-01, I saw data for the previous day!
Eventually I figured out that this only happened when I set my beginning of day to 6:00 am. When I removed the beginning of day, it worked as expected.
Explanation: I'm almost certain what's happening is that when I apply my date filter, say 2010-04-01, HM applies that as 2010-04-01 at midnight. So then it filters out the hands on the same "day" (HM day) as 2010-04-01 0:00 -- but if I didn't start any tourneys between Tuesday 0:00 and Tuesday 6:00, then filtering for say Tuesday is really only including tourneys for Monday.
Suggested behavior: When I set my start of day ("Set day starts at") to 6:00, it's because I occasionally start a tournament just after midnight before going to sleep for the day. I daresay quite a few poker players keep a schedule similar to mine, perhaps playing for as much as a few hours after midnight then sleeping from, say, 02:00 to 10:00 local time.
Just changing the definition of the specific day filter, say to 23:59 on the specified day, would fix the problem for us.
However it could introduce a new problem for someone who does the opposite, who sets their start of day to 18:00 because occasionally they start a tourney at 23:00 Monday that they really consider to belong to Tuesday. Again, I daresay there are relatively few people in this situation, waking up at say 21:00 local time and starting their poker day. (There might also be some users who track in Full Tilt time or some other site-specific time zone instead of local time.) These users need the present functionality preserved so that specific day filters pertain to 00:00 of the specified day.
If I had to default to only one as a quick fix, I'd change the single date filter to assume 23:59. But for a long-term fix I'd suggest having a configuration option somewhere to set this.