Here's a Greasemonkey / Tampermonkey script that'll run a stop watch until an edit reaches 5 minutes of age. Give it a try and see what you think. It might have bugs, but it's close enough for government work I think. (That's a United States idiom meaning it's poor quality, but it works well enough.) Bear in mind that it only works on new posts and post edits, not on comments.
// ==UserScript==
// @name SE realtime dates
// @namespace http://stackapps.com/
// @description show Stack Exchange timestamps as realtime counters
// @include /^https?:\/\/(\w+\.)?(stack(overflow|exchange|apps)|serverfault|superuser|askubuntu|onstartups|mathoverflow|mso)\.com\/.+/
// @version 1
// @grant none
// ==/UserScript==
function setIntervalWithContext(code, delay, context) {
return setInterval(function() {
code.call(context);
}, delay);
}
function startTimer(el) {
if (!el.title) return;
this.el = el;
this.interval = setIntervalWithContext(function() {
if (!this.el) return clearInterval(this.interval);
var x = Math.round((new Date() - new Date(this.el.title.replace(' ','T'))) / 1000);
if (x>299&&x<360) this.el.innerHTML = '5 mins ago';
if (isNaN(x) || x > 299) return clearInterval(this.interval);
var m = Math.floor(x / 60) % 60,
s = Math.floor(x % 60),
s = (s < 10) ? '0' + s : s;
this.el.innerHTML = m+':'+s+' ago';
}, 250, this);
timers.push(this.interval);
}
function addTimers() {
while (timers.length) {
if (timers[0]) clearInterval(timers[0]);
timers.splice(0,1);
};
var timestamps = document.getElementsByClassName('relativetime');
for (var i=0; i<timestamps.length; i++) {
var stopwatch = new startTimer(timestamps[i]);
}
}
var timers = [];
addTimers();
addEventListener('click', function() { setTimeout(addTimers, 1000) }, true);