Summary
I have a problem with a dashcard that should be filtering the cards for which it's aggregating statistics to include all cards due "today." Cards due at or after 9:00 PM, however, are excluded from the search.
Have I found a bug? Am I misunderstanding a feature? Or am I just confused?
The details
I have a board on which I've set up dashcards that filter for cards due today, sum over a custom field (completion time estimate), and display the total of all time estimates for cards with a due date of today. In theory, this should mean that I can see the total amount of time tasks due today are expected to take.
Here's the dashcard's filter configuration:

And a screenshot of the aggregation configuration:

When I have a card with a due date at 8:59 PM, it's included in the aggregate total.

But when I change that card's due date to 9:00 PM, the aggregate total shown in the dashcard is decreased by the value of the custom field over which I'm aggregating:

Hypothesis: maybe this is a time zone problem?
I thought that maybe this is an issue with time zone settings. But when I go to check the Atlassian account time zone settings, everything seems to be in order:

I traveled recently, and I made sure to update my Butler account's time zone settings away from, then back to Pacific Time after I traveled, hitting the "Set Butler time zone..." button:

And I checked the computer's settings as well, just for good measure. Windows says the same as the rest of my settings:

So where next?
I'm stumped. I've got what seems like a simple "due today" filter, and it's not quite working for me. It's filtering in Trello cards for which the due date is yesterday with a time of 9:00 PM or afterward and it's filtering out Trello cards for which the due date is today with a time of 9:00 PM or afterward.
Any ideas? Have I found a bug? Am I misunderstanding a feature? Or am I just confused?
(N.B. I have also raised this as a support ticket, TRELLO-141572.)