It isn't unusual to see a buffalo nickel with the date worn away, because the date was printed on a raised part of the coin's design. Since determining the age and rarity of the coin is impossible without the date, such coins are usually of little value to coin collectors.
No buffalo nickels were ever minted without a date. If the date is not visible, that means it has been worn away by time and use. Sometimes it is possible to determine the date of a worn nickel by putting a drop of ferric chloride on the spot where the date was. On a buffalo nickel, the date is printed on the obverse side on the Indian's shoulder. Ferric chloride may make the date visible temporarily, but it will also leave a rough, blotchy acid mark, rendering the coin valueless to collectors.