So do I, but I don't think I need to worry too much about confusing them with 2090.

Here you go gramps:

(shortD) => {
    return parseInt(shortD.slice(0, 2), 10) > 50 ? "19" + shortD : "20"+shortD;
}

are we just talking digital because i've inherited archives. my current one only goes back to the 1950s but in the next decades i expect to get some going back centuries.

The exact date of creation is usually preserved in the filesystem, we're just talking about what to name the documents themselves. The filename should be short and to the point, it gets truncated if it's too long, and on windows you only have 260 characters for the entire path to the file plus the name.

We're just talking about the filename, the exact creation time is tracked by the OS. Plus I'd imagine most documents also have a time and date inside. The file name is mostly for sorting and human readability.