So basically what you want to do, is let the user enter two dates. You'll then want to adjust those dates as follows:
The start of the day should be automatically taken care of when the user enters a date (assuming you use a standard datepicker), the end of the day can be achieved by adding one day to the datetime “addDays($date, 1)” and then subtracting one second from that “addSeconds( addDays($date, 1), -1)”.
Now you can write an xpath where the business date is >= the startdate and <= the enddate. That should give you all objects that occur between those dates without looking at the specific time of the object.
When using: '[%CurrentDateTime%]' XPath will check against current day and not the time.
[Business_Date = '[%CurrentDateTime%]']
This should work for your retrieve.
If for some reason it does not please have a look at the documentation all Xpath Constraint functions can be found here: