]...Since they had no watches. How did people measure time? By looking at the sun probably.

After Darcy's second proposal, he & Elizabeth continue to walk and talk. Then,

"After walking several miles in a leisurely manner, and too busy to know any thing about it, they found at last, on examining their watches, that it was time to be at home."

So, it appears to me that JA is saying that both Elizabeth & Darcy have watches. Must have been fairly common, then, amongst the gentry.