The best answer to this question is "usually not". Many factors could cause a discrepancy between a scheduled build date and an actual assembly date, such as shortages of parts, worker strikes, and any number of other things. From what we've seen, the actual assembly date tends to differ from the scheduled build date. As an example, we know that the scheduled build date for the start of 1969 model year production at the Dearborn, Metuchen, and San Jose assembly plants was "04G", or July 4, 1968. According to Marti Auto Works, the actual production date for the first 1969 Mustangs assembled at Dearborn was August 16, 1968 (43 days behind schedule), and it was the first plant to start assembling Mustangs for that model year. We believe that Metuchen started production on August 20th (47 days behind schedule) and San Jose started production on August 28th (55 days behind schedule) based on information documented on Marti reports.
It's interesting to note that cars with a scheduled build date of "04G" are commonly introductory show units. This date code appears to be more of a symbolic placeholder than an actual scheduled assembly date because July 4th is a federal holiday in the United States, and the assembly plants are normally closed.