readtime/readtime.js

var wpm = 200; // Average reading rate in words per minute (WPM)

var ps = document.querySelectorAll('p');
var wordCount = 0;
for (var i=0; i<ps.length; i++) {
	wordCount += ps[i].innerText.split(/\s+/).length;
}
var $time = document.querySelector('#post-body .dt-published');
if ($time) {
	$time.style.display = 'inline-block';
	$time.insertAdjacentHTML("afterend", '<span style="color: #666"> &middot; ' + Math.round(wordCount / wpm) + ' min read</span>');
}