Your observation is correct, per default the DFWP filters by date only.
I have no way to check this out at the moment, but out of the top of my
hat, you need to do the following:
- switch to Code view
- find the data source
- open the tag property task pane
- and change the selectcommand CAML from
<Value Type="DateTime"> to
<Value Type="DateTime" IncludeTimeValue="True">
for all date fields that needs to be filtered by date/time
If you need more info Google up IncludeTimeValue="True", it was blogged
before.