You Can't Date a Product by Its Patent

Over 6 million utility patents have been issued since 1836. Because they are numbered sequentially, it is possible to get a rough idea of the age of an item by determining when a utility patent number was issued. This comparison, however, gives the lower limit on the age of an item. This is because products often remain on the market for years with no change in how they function even as their appearance changes repeatedly. For example an electric percolator introduced in the 1950s may operate based on the same technology used in a 1920s percolator. As a result it may display the same utility patent as the 1920s model.

Since 1843, over 418,000 design patents have been awarded. Design patents are preceded by the letter "D" or the letters "Des." Design patents provide a more reliable method of dating an item because the patent covers the ornamental design of an item. For example, electric clocks produced in 1925 and 1935  may be covered by the same utility patent, but the case designs may be covered by patents issued 10 years apart. Even the use of design patent numbers to date items, however,  can be problematic. Popular designs often remain on the market for 20 or more years.