While this neck dating is useful in roughly determining the age of a guitar, it is certainly not definitive.
The neck date simply refers to the date that the individual component was produced.
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.
Have a read through and hopefully you will pickup a few things to better assert yourself in the late 70's Stratocasters buyer's market. M = Model or Manufacturer O = Operator N = Neck configuration W = Week Y = Year D = Day Neck Stamps: MMNN*WWYD Example: 0900*3893 - Found on the very end of the neck heel, if at all, in green or dark red ink.
First four digits are paired up, 09 is the model number for the Stratocaster, and 00 is the neck configuration, in his case a fretted Maple neck with a Rosewood fingerboard. 38 is the week, 9 stands for the year, 1979, and 3 is the day of the week, which is Wednesday.
Neck-dating can be useful in determining the was produced, rather than the complete instrument.
Given the modular nature of Fender production techniques, an individual neck may have been produced in a given year, then stored for a period of time before being paired with a body to create a complete guitar, perhaps, for example, in the following year.