That's the original date on the file, but the comment being discussed (and much of the code in that function, it seems) was from 17 years ago. See the git-blame:
See the text of the comment: "12/12/91". The `git blame` is likely due to not having imported the previous history as commits when the original import to the git repo happened. I worked at a company that moved from VSS to TFS for a ~10 year code base. It took months for the consultants to get the import to preserve the history. Too bad this didn't happen in this repo.
edit: Found this [0]
Initial git repository build. I'm not bothering with the full history, even though we have it. We can create a separate "historical" git archive of that later if we want to, and in the meantime it's about 3.2GB when imported into git - space that would just make the early git days unnecessarily complicated, when we don't have a lot of good infrastructure for it.
Let it rip!
It's a weird feeling where if you work on this code, it must be like living archaeology, digging up remains, but at the same time sculpting new remains for the next generation. The other profession that must be like this is Law. In some ways this file feels more permanent than the blockchain. Although someone might come refactor it to Rust one day! If you do keep the comments at least!