I’m back with some success I’ve achieved in geotagging in darktable for Windows, thanks to the information in your previous message, houz.
I selected all the photos in a set corresponding to a GPX track. I then opened the “geotagging” module and set the “time offset” to +05:00:00 and clicked on the check mark. This added 5 hours to the displayed “datetime” of each photo. This 5 hours corresponds to the difference in the camera clock setting and UTC. (America/Chicago with Daylight Savings Time as described in my previous message.)
Then I clicked on “apply GPX track file” and, in the dialog box that opened, I selected the “camera time zone” as “UTC”, selected the GPX track file, and clicked “open”. All the photos were correctly geotagged and verified in the map display.
So it appears my problem was due to what you said: “Selecting the time zone doesn’t work on Windows, it always uses UTC.”