Timex did not use serial numbers on their vintage watches. From 1963 onwards a string of numbers may be found at the bottom of the dial on mechanical watches, the last two digits refer directly to the last 2 digits of the date. This string may be seperated into 2 groups by the 6 marker, its the last 2 of all digits that will tell you the year. This code is often very small and easy to miss.
From 1985 onwards Timex put a date code on their watches, it is 2 characters long made up of letters or numbers and can be found on the case back. That is the code this tool decodes. As the code was repeated over the years you may be returned two possible dates, visual indications and the production run dates can then be used to narrow it down more.