diff --git a/lib/sitemap.js b/lib/sitemap.js
index df1e135d..211f7109 100644
--- a/lib/sitemap.js
+++ b/lib/sitemap.js
@@ -528,6 +528,7 @@ function createSitemapIndex(conf) {
  */
 function buildSitemapIndex(conf) {
   var xml = [];
+  var lastmod;
 
   xml.push('<?xml version="1.0" encoding="UTF-8"?>');
   if (conf.xslUrl) {
@@ -542,10 +543,21 @@ function buildSitemapIndex(conf) {
     xml.push('<sitemapindex ' + conf.xmlNs + '>')
   }
 
+  if(conf.lastmodISO) {
+    lastmod = conf.lastmodISO;
+  } else if(conf.lastmodrealtime) {
+    lastmod = new Date().toISOString();
+  } else if(conf.lastmod) {
+    lastmod = new Date(conf.lastmod).toISOString();
+  }
+
 
   conf.urls.forEach(function (url) {
     xml.push('<sitemap>');
     xml.push('<loc>' + url + '</loc>');
+    if(lastmod) {
+      xml.push('<lastmod>' + lastmod + '</lastmod>');
+    }
     xml.push('</sitemap>');
   });