It varies by the component, not a standardized one size fits all for every part, for example I believe engine blocks were dated by a 10,000 day calendar at least for castings.
But dates on other components have different formats which were likely dependent on what supplier was used. Wiper motors were typically 4 digits being the day of of the 365 day year and one digit for year.
Big blocks have the actual date cast on the side and the actual date stamped on the ID pad. Small block engines use the actual date on the side but then use the 10,000 day on the assembly stamp. I'm not really up on what other engines used. The FSM and the GG book cover some of this. The master parts book also has some info. Other than that you just have to scratch around and see what you can find or figure out.
