I have a solution for Firefox users that uses userContent.css and is probably adaptable to other browser. There's a greasemonkey script in the same place too.
I added the code to a gist.
The solution comprises using CSS to get the title attribute from the date span tag, and display it on the page. The span title uses what I think is RFC 3339 format, very similar to ISO-8601. As there is a space, just the YYYY-MM-dd thus can be shown, comments show a full ISO-like date.
The original date is just hidden by matching the background colour. Could use color:transparent, and probably do something fancy like float the pseudo-element ::before over the actual <span>.