They are fairly consistent but will vary over time and would not be the exact same every year. It depends on the time scale you choose. The differences are small in the year to year. Main thing to check out is precession, it takes the earth 26,000 years to complete one precession, part of the yearly difference you see. Also things like large earthquakes tend to speed up the rotation, think a figure skater pulling their arms to their body during a spin.

Another thing I would consider is the fact time zones didn't exist back then like now, not sure you could confirm their 8 and 10 pm were in fact the same 8 and 10 pm they experience today as time zones didn't exist until after the 1880s.

Also the time they would need light will vary between those two times in the evening as they represent the extremes between the two seasons. So at the height of summer your good until 10pm, the bottom of winter you need it at 5pm, but in the middle of fall you might need it at 7:30pm.