Don’t worry about UTC. If you’ve geo-located your model to OKC, UTC should show -06:00 That is the time offset from Greenwich. As I type it is about 5:45 PM here so it would be 11:45 in Greenwich in the UK. The US and UK adjust clocks on different days. If we were in between and the shadows were that critical, you could change the UTC setting to match the current offset. Or if you want to show them what the shadows would look like in Arizona or Oslo, you could make an adjustment to the UTC offset.
Maybe you should see if your client would like to move to Jetmore. Real Estate is much less expensive there. 