I'm a long way from a (copyright) attorney, so I really don't know what I'm talking about on the finer points.
In general, of course, the act of creating is what currently confers copyright. So if you write some code, for example, you automatically hold the copyright. No need to do anything official or call a lawyer.
But you're correct that there are exceptions for older works that were never published or formally registered under the old (pre-1978 system). So yup, my (pretty much worthless) opinion agrees with your reading that it would be 70 years after 1983. And who knows, the Mighty Mouse might might well rise up again as 2024 approaches (the current date when Mickey Mouse will otherwise enter the public domain).
In general, of course, the act of creating is what currently confers copyright. So if you write some code, for example, you automatically hold the copyright. No need to do anything official or call a lawyer.
But you're correct that there are exceptions for older works that were never published or formally registered under the old (pre-1978 system). So yup, my (pretty much worthless) opinion agrees with your reading that it would be 70 years after 1983. And who knows, the Mighty Mouse might might well rise up again as 2024 approaches (the current date when Mickey Mouse will otherwise enter the public domain).
But