There were periods of time when this was not consistently done, (between 19), and there are certainly other examples of short periods of time, and individual pieces, where the dating was simply omitted.While this neck dating is useful in roughly determining the age of a guitar, it is certainly not definitive.
The Fender serial number decoder currently supports all documented MIA, MIJ, MIM, MIK and MII formats with the exception of Custom Shop, Relic and Reissue instruments.
Please note that fender serial numbers tend to overlap by at least a year, and thereby the date of your guitar can only be approximated.
The neck date simply refers to the date that the individual component was produced.
Given the modular nature of Fender's production techniques, an individual neck may have been produced in a given year, placed in the manufacturing warehouse and remained in stock for a period of time, and then subsequently paired with a body to create a complete guitar in the following year.
The following chart details the Fender serial number schemes used from 1965 to 1976.