A 'date nail' was (is?) used by the railroad to determine when to replace railroad ties.
The idea first started in 1890. At that time, wooden ties when left out in the open,
where else for a railroad?, would only last about 10 years before they
had to be replaced.
One idea, well, not a solution as such, was an aid. They created a special nail whos
sole purpose in life was to identify the year the railroad tie was set in the roadbed.
I guess the first nails had a '90' on the head. These are hardened nails
so that pounding them in didn't flatted the year code (much).
I found out about this when I worked at a locomotive manufacturing plane in La Grange,
IL back in the early 80s. Ever since that time, I can't go near train tracks without looking
for date nails. The train tracks that run thru Rose Canyon where I live
are no exception. They are dated '44', the year I was born. That makes these timbers 58 years
old. Why didn't they rot? Creosote - a black fowl smelling muck that
protects them. In fact, railroads stopped using date nails around 1970.