I'd thought the 1940's, but everything that I'm finding there is mid-late 1800's
Zero modern trash.
That's why I don't understand why this post is out in the middle of nowhere.
It is set into a very large section of badly decayed concrete.
In 1836, Sorel in France took out the first of numerous patents for a process of coating steel by dipping it in molten zinc after first cleaning it.
Galvanized pipe replaced lead piping in the early 1900's.
Galvanized piping can last up to 70 years depending on the thickness of zinc in the original galvanization.
Ok, enough history....
I would put your post no earlier that the 1930's by the amount of rust showing through the zinc coating.
Click the most recent year on the maps, then click the little flashlight icon, this will open up another menu on the right where you can choose the older topo maps. As you hove over the area with your mouse it will show through the new map to the old topo. (Assuming of course there are maps for the area).
What is this a picture of?
Is the photo in the correct orientation (I assume it is, as it seems to be weathered / rusted more on "top", but just wanted to confirm).
Some photos post sideways here.
Can't help yet with age or purpose.
It looks like barb-wire to me. I recall seeing something somewhere that you could estimate the age of barb-wire by determining the manner of construction (type of barb, twist rate, intervals between barbs, etc...), so maybe that approach could yield some clues.
Seriously though, as the other guys have already mentioned... context and perspective is everything when it comes to photo's.
I too, will often study fence posts, property lines, wind rows and even drainage ditches on rural properties to help determine age and past uses.
Your fence post is definitely unique in design, but often older posts such as this one, were added to or modified over the years depending on how the property's uses changed.
I'm thinking your post dates to the 1920s - 40s based on the square nut and the style of the finial.
I look forward to seeing more pics of the fence line and the attachment hardware.