It's possible to find out exactly in what year and month the seiko watch that you have is produced, because Seiko has a very well-structured serial and reference combination system.
In the back of Seiko watches there is a range of information presented. This can tell you the movement type, date of manufacturer, how to open the case, which company produced the product, what the case is made from and the specific case design code. These codes are also occasionally stamped on the inside of the case back while the external surface remains clean.
The details on the back of Seiko watches have changed over the years but much of the general information presented have been consistent since the early 1960’s.
The information on case backs were detailed in the 1969 Let’s Go Suwa No. 160 internal company magazine. This was a publication for the Seiko Suwa factory that was produced from the late 1950’s to the 1970’s. In the article they describe the different details that were shown on the case backs at that time.
Serial Number: The serial numbers on modern Seiko models are comprised of 6 digits. The first digit indicates the year, the following character the month and the remaining 4 digits a unique production number. The month is listed as 1-9 for January to September, then 0 for October, N for November and D for December.
For example on this case number of 7005-8062 the serial number is 652290. This indicates the example was manufactured in May 1976 and the unique production number is 2290. Exactly how this production number was allocated between models and production runs is not clear.
As the year is only comprised of the single digit, to determine the relevant decade the model is from you need to know the period that the watch was released. This can be determined by the calibre number, as most movements were not produced for longer than a decade. For movements that were produced for a long period of time they often go through various revisions and are marked with a version letter like A, B, C etc at the end of the general calibre e.g. 7S26C. Other clues to the model decade can be the watch styling or branding, for example Seiko Advan models are associated with the 1970’s.
The six digit serial numbers are found on Seiko models beginning from 1966. Previous to this Seiko used a seven digit serial number with a similar format. This seven digit serial number continued until 1969, so there is a transition period where some models had a six digit serial and other models a seven digit. While most of the seven digit serial numbers followed the Year, Month, Production # format there are some examples that seem to follow a Month, Year and Production # format.
If you want to confirm the production date on a specific model there are a number of online calculators available. You just enter your watch serial number and caliber number and the date will be calculated. A comprehensive date calculator can be found here - SEIKO - Serial Number to Manufacturer Date Decoder. There are some errors on this site but in general it is quite reliable.