Which country will still use banknotes, coins, or any other form of currency that is issued by the government and not a barter system, that could still be used to this day?
Thank you for your time.
This is a bit of a difficult question to answer due to the way money has evolved over time. The British like to claim that the pound is the oldest currency in the world still in use since the penny was introduced back in the pre-Saxon days, but the Bank of England itself only came into being in the late 17th century, and the modern pound was really created on Decimal Day in the 70s. And a good thing, too. Those pre-decimal pennies are LETHAL.
The Russian rouble is the oldest decimal currency still in use (although it has, of course, gone through a few governments,) as it was standardized and decimalized by Peter the Great in 1704.
If you want to include non-traditional currencies, cowry shells win hands down. They still serve as currency in some regions of the pacific and may possibly be a legal medium of exchange in Papua New Guinea. I've seen conflicting reports on the matter.
Swiss Francs coins haven't changed in design since the mid- to late-1870's. The 2Fr. coin introduced in 1874 looks identical to the one minted in 2014. http://www.schweizer-geld.ch/federal-coins-2-francs/en/13-1
I am pretty sure that paper currency issued by the United States remains legal tender no matter how old, but I have not found an explicit statement in so many words. A Treasury Dept. FAQ at Legal Tender Status implies it, by mentioning that no currency from before 1933 is redeemable in gold (but doesn't say that it has no value), and Buying, Selling, & Redeeming has "You may redeem the [gold certificate] notes you have through the Treasury Department or any financial institution.". If someone can find a more definite answer, I'd appreciate it.
But this doesn't quite answer the question's "in circulation", "issued by the government", "could still be used". For example, denominations of over $100 exist but are withdrawn by the Fed when they fall into their hands (Denominations), and I think that gold certificates, silver certificates, and US Treasury Notes are too. Further, if you presented (for example) an Educational Series $5 from 1896 at the local convenience store, I doubt that it would be accepted by the clerk.
But I have very occasionally received in circulation silver coins (pre-1965) and Treasury notes (before the 1970s), but their sizes and designs were very similar to recent ones.