diff --git a/package.json b/package.json
index 286eadfd3a..ce2a99f88c 100644
--- a/package.json
+++ b/package.json
@@ -26,7 +26,7 @@
     "gm": "1.14.2",
     "gravatar": "1.0.6",
     "nconf": "~0.6.7",
-    "sitemap": "~0.7.1",
+    "sitemap": "~0.7.3",
     "winston": "~0.7.2",
     "rss": "~0.2.0",
     "request": "~2.34.0",
diff --git a/src/sitemap.js b/src/sitemap.js
index 10db5e0032..7ce531ae56 100644
--- a/src/sitemap.js
+++ b/src/sitemap.js
@@ -7,6 +7,7 @@ var path = require('path'),
 	nconf = require('nconf'),
 	categories = require('./categories'),
 	topics = require('./topics'),
+	utils = require('../public/src/utils'),
 	sitemap = {
 		getStaticUrls: function(callback) {
 			callback(null, [{
@@ -55,6 +56,7 @@ var path = require('path'),
 						data.topics.forEach(function(topic) {
 							topicUrls.push({
 								url: path.join('/topic', topic.slug),
+								lastmodISO: utils.toISOString(topic.lastposttime),
 								changefreq: 'daily',
 								priority: '0.6'
 							});