As of version 2.5.3 we have added functions that allow you to extract a certain part, e.g. the hours, from a DateTime value in an XPath constraint.
Here is an example that retrieves all objects that have the StartTime attribute set to 8am:
[hours-from-dateTime(StartTime) = 8 and
minutes-from-dateTime(StartTime) = 0]
For the full list of functions, see this documentation page.