se_grace_timer.user.js

// ==UserScript==
// @name         SE realtime dates
// @namespace    http://stackapps.com/
// @description  show Stack Exchange timestamps as realtime counters
// @match        *://stackexchange.com/*
// @match        *://*.stackexchange.com/*/*
// @match        *://stackoverflow.com/*/*
// @match        *://*.stackoverflow.com/*/*
// @match        *://stackapps.com/*/*
// @match        *://serverfault.com/*/*
// @match        *://superuser.com/*/*
// @match        *://askubuntu.com/*/*
// @match        *://mathoverflow.net/*/*
// @version      1.2.1
// @downloadURL  https://github.com/calraith/gm_scripts/raw/master/se_grace_timer.user.js
// @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 = (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 || x < 0) 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) clearInterval(timers.pop());
	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);