A model year (hence the date code on your cowl tag) began the last week of August or early September of the year preceding the model year of the car. They were built until late July or early August when the factory began to set up for the next year's model.
We refer to the first year of the second generation Camaros as 1970 since this is what GM called them when new. Since they were introduced mid-season (February), hence being called 1970 1/2 by some, it is not wrong to call them this. (All of the last year of the first generation cars were called 1969 even though they were built into the 1970 model year, which adds more credibility to 1970 as opposed to 1970 1/2.)
The 1977 Z28 was a mid-year reintroduction.