Thanks Tom and others who responded.

The answer was easier than I thought. I looked up the solar time for Chattanooga and found that it is 1 hr and 35 minutes "behind" EDT. One hour is due to daylight savings time and the additional 35 minutes is due to Chattanooga's relative position within the Eastern Time Zone. Of course, in the 1860s there was no 'standard time' and no time zones. A person could set their watch fairly accurately to solar time if there was a properly installed sundial. With a sextant and a compass and the know-how, an officer or adjutant could determine the local solar time.

So, assuming the officer was fairly close to precise local solar time, he stated that at 7 PM it was completely dark. We were there on the same date, Sept. 19th, and it was 7 PM EDT and well before sunset -- but the solar time was really 5:25 PM solar time. Sunset on Sept. 19th at Chattanooga is 7:42 PM EDT which is 6:07 PM solar time. So, when the officer said it was 'after dark' at 7 PM, that was 8:35 PM EDT and 53 minutes after sunset in heavy woods. It was certainly well after dark!

So when the ranger said "their clocks were off by 1 1/2 hours", their clocks were not 'off', they were just using a different measurement system, i.e. local solar time versus our arbitrary UTC system.