If 15 degrees is one hour, then 10 degrees is 40 minutes -- the difference between -75 degrees and -85 degrees. On Sept. 19th, solar noon at Chattanooga is 1:35 PM EDT. So, when the clock in my phone said 7 PM EDT on Sept. 19th, it was 5:25 PM solar time. If the officer's watch was set to solar time and said 7 PM, it was 8:35 PM EDT.

For clarification, the officer described the passage of time from dusk until completely dark. I was paraphrasing his comments about darkness and 'after dark'. He was clearly speaking of the transition from dusk to 'pitch black' in the heavy woods. I'd have to do some searching to identify who the officer was and find his report, I'm recollecting (poorly) from a ranger-led tour on the battlefield.