You can make the absolute date appear for each timestamp on a GitHub page by injecting this CSS onto the page, manually or with a browser extension such as Stylus:

    relative-time::before {
        content: attr(title);
        margin-right: 0.5em;
    }

The result is that all relative times have their absolute time prepended, like this:

> Nov 20, 2017, 2:11 PM EST an hour ago