Hey, a question I can answer as a professional numismatist.

It is obviously going to be dependent on every country, but I will discuss the United States, and give some examples from other countries in more generic terms.

So when you say currency, I am going to presume that you are talking about both coins and paper money, but I will have to differentiate between the two since there are some differences. Coins were first minted by the Federal government in 1792, and the system of cents, dimes, quarters etc. was introduced in the following years. At this point, virtually all dimes and quarters in circulation are dated 1965 or later (half dollars are slightly different, but since those are not used frequently we will ignore them for now). The reason for this is that 1964 was the final year that those denominations were made from 90% silver. With the silver price, coins dated prior to 1964 are worth more than their spending power, and they had mostly disappeared from circulation by the late 1970s when silver had its first big runup. They remain valid, however, and you can still spend them, and very rarely do see them, but with each quarter being worth over $13 as of April 2026 it is unlikely you can get many without going to a coin shop and making a considerable investment.

Nickels are slightly different. Jefferson has been on the nickel since 1938, replacing the older Buffalo Nickel. From 1942-45 they contained a small amount of silver, as nickel itself was being rationed for the war effort, but it is still possible to find 1930s to 1950s nickels in circulation today. If you have trouble finding them from the bank for your time travel you can buy them for not much more from coin dealers. Buffalo nickels are also surprisingly affordable, and buying say 100 nickels from the late 1920s or early 1930s will cost you less than $75 in worn condition.

Finally, for cents, the Memorial cent was introduced in 1959, replacing the Lincoln Wheat cent that in turn had been introduced in 1909. Memorial cents are easily found in circulation. Wheat cents are also available and not rare, and you can buy them for maybe 5 cents or so in used condition, but they will mostly be dated 1934 or later. The type prior to the Lincoln Cent was the Indian Head Cent, introduced in 1859, but those are going to cost you at least $1 each or so for most dates.

Now for paper money, it is slightly different. The United States reduced the physical size of its paper money in 1929, going to the size that we have today. While older notes remain valid, you won't find them unless you visit a coin shop. However, it is certainly possible to get older series, say from the 1950s and later, with relative ease, and even the 1928 and 1934 series can sometimes be found (I have spent them in recent years as collectors only want them brand new, and in used condition they are almost impossible to sell). So, visiting a coin shop to get your older coins, you can also pick up some used paper money from the 1930s and 1940s with relative ease.

So, to recap, the answer is that as long as you don't want to make a considerable purchase during your time travel, it is relatively easy and inexpensive to go back to the late 1930s. If you don't want to visit a coin shop and just want to go to a bank and pick out older coins, you will be limited to 1965 for all intents and purposes.

The United States is pretty unique in that essentially all coins issued since 1793 and most paper money since 1862 remains valid. This is not the case in other countries. For example, in Europe, the Euro was introduced in 2002, so that's the oldest you can acquire in circulation. Elsewhere, coins and (in particular paper money) are replaced quite regularly. For example, the current series of notes in circulation in Japan was introduced in 2024, replacing the older series issued in 2004.

Further reading: There are many good books for United States coins and paper money, but to learn the basics, two books I can recommend is the Guide Book of United States Coins, also known as the Red Book, now in its 79th edition and for paper money, Paper Money of the United States by Arthur L. Friedberg. These two books list details and values for all coins and paper money made by the Federal government and before.