How does slapping a copyright date on every file affect that? AFAIK the file itself is covered under the copyright of the whole work anyway and doesn't even ne

How does slapping a copyright date on every file affect that? AFAIK the file itself is covered under the copyright of the whole work anyway and doesn't even need a copyright notice. If the file itself does need a copyright notice, then how does updating the date continue to perpetuate the copyright an additional year into the future with no further updates to the text? AFAIK republication of a book with a new copyright date does not push copyright dates into the future. Even if you update a few lines in the file, again AFAIK that doesn't update the expiration timer for all the rest of the code. My understanding is that incrementing the data does nothing for the copyright expiration of the file, and that the act of updating the file with new lines of code is what protects those lines of code into the future, and that no copyright notice is even required. It isn't like trademark where failure to notify and defend your trademark can eventually lead to your losing your trademark. Since 1989 copyright notices have not been required under law, but the purpose of a copyright notice is simply to reduce the possibility of an "innocent infringement" defense.