I think what most people will find unhelpful about how CUIC works, in this context, is that if you pick 1 day from 8:00am to 10:00am, the results are pretty clear how many calls you took in that 2 hour window, for that one day. However, if you wanted, in the same exact report, to see that, on a day by day basis, for say a week (filter "Last Week"), the report will just add up all of the calls, from each day, within that 2 hour window, and show you the total. I'd wager most people want to see it broken down per day.
E.g., This kind of report would be more helpful, than a total
Sunday from 8:00am to 10:00am 0 Calls Presented
Monday from 8:00am to 10:00am 63 Calls Presented
Tuesday from 8:00am to 10:00am 22 Calls Presented
Wednesday from 8:00am to 10:00am 87 Calls Presented
Thursday from 8:00am to 10:00am 34 Calls Presented
Friday from 8:00am to 10:00am 56 Calls Presented
Saturday from 8:00am to 10:00am 4 Calls Presented
To recreate this yourself, just run the Stock Contact Service Queue Activity Report for 1 day from 8:00am to 10:00am, and then switch it to Last Week, but keeping everything else the same. You'll see that you end up with the same report format, just more numbers in each column. Most likely, if the report runner is asking for a custom time range, they're wanting to understand what happens in that time period on each day individually, and not collectively.
The only way I know how to solve this, is to run 7 reports, one for each day, which is a pain. Hopefully there is a better way. Know of one?
PS, I'm not positive this is the OP's problem, but it's likely.
EDIT: Looking at the OPs post closer, it might not be what I just described above, and in fact could just simply be this:
If you pick a custom time range for a report filter, call it 8:00am to 10:00am, and you pick a date range of more than 1 day, then the first day in the date range is matched with the start time (8:00am) while the last day in the date range is matched with the end time (10:00am).
Example:
If you pick 3/1/2019 - 3/2/2019 and 8:00am to 10:00am, then your effectively reporting on calls from 3/1/2019 @ 8:00am to 3/2/2019 @ 10:00am (a single 26 hour window of call activity, not two 2 hour windows).
You can verify this yourself, by running that same kind of filter, and then clicking the "i" icon (near the help button) which will show you something like this:
Filter Information
Generated Time: 2019-03-07T10:28:29-06:00
Username: CUIC\uccxadmin
No of Records: 47
Start Day Of The Week:
Sunday
Logged in User:
CUIC\uccxadmin
Interval [Starting At]:
03/01/2019 08:00:00
Interval [Ending At]:
03/02/2019 08:59:59
CSQ Names:
MyTestCSQ
Interval Length:
Sixty [60] Minutes
EDIT2: I think I found a report which does this
Check out Contact Service Queue Activity by Window Duration
