Us patent dating chart
The United States patent number for each year is the first patent issued that calendar year.
A patent date is when something was patented, not when it was actually made, many items were manufactured for many years after it the patent was granted.
For example, if you have an antique widget that has stamped on it "" you can look at the chart below and determine that the earliest date of the item would be about 1917-1918, and had a term from 1917 to 1934 (17 Years), so that is the approximate window of time of your widget.
The number for each year is the first design patent issued that year. Design patents refer to any new ornamental design for an item.
The design patent protects only the appearance of an article, but not its structural or functional features.
Since some patents will have multiple classification numbers, you can widen your net by searching with these numbers.
You can also obtain classification numbers at "Tools to Help in Searching by Patent Classification" at step 3.