An overdate occurs when a coin exhibits dates of different years. This can occur when the date is punched or hubbed into a die for one year and the date of another year is punched or hubbed over one or more of the digits of that working die. This might have happened intentionally for a number of reasons:

1. There might have been a shortage of dies or metal to make dies.
2. The mint may have made too many dies for a particular year, with many dies left over at the end of the year.
3. The working dies produced for one year might not have been used or may have been used very little.
4. On the second hubbings of a working die, a new working hub with a different date was used.

In the 18th and 19th centuries at the Mint, the date was punched into the working die by hand. It was an acceptable practice during the 19th century to reuse new or slightly used dies from one year to the next to get the most out of each die. To reuse the die for the next year, the engraver could remove the old date with abrasives and punch in the new date or just punch the new date over the old. Any remnants of the old date that were left, especially of digits that were different, would produce an overdate.

An overdate could also have happened by accident. The engraver could have picked up the punch for one year and when he went to strike the date punch a second time he picked up the punch for a different year. Even though an accident like this is unlikely, it is possible, especially when dies were being hubbed around the New Year and punches for more than one year might have been lying around.

Another way an overdate might have happened by accident was when a die was hubbed at the end of the year. The working hub of one year was used to hub the working die, and then when the die was returned to be rehubbed after annealing, a working hub of a different year was used to hub the die. This happened for the 1918 over 1917 Buffalo nickel and the 1942 over 1941 Mercury dime.

Below on the left are photos of the 1880-CC Morgan Dollar overdate VAM-6 with an obvious 7 seen in the lower and upper loop of the second 8. The two photos on the right are of the 1887 Three cent nickel 8/7 which is found on both proof coins and business strikes. This variety is especially nice because it also has 4 digits in the denticles.