Adds <time> with datetime attribute to allowed post tags.

gce-alter-allowed-tags.php

<?php
/*
Plugin name: Add time to allowed tags
*/

function gce_alter_allowed_tags() {
	global $allowedposttags;

	$allowedposttags['time'] = array(
		'datetime' => true
	);
}

add_action( 'plugins_loaded', 'gce_alter_allowed_tags' );