diff --git a/theme/image.php b/theme/image.php
index 2ec860dc1bd..cac1765bd22 100644
--- a/theme/image.php
+++ b/theme/image.php
@@ -71,13 +71,12 @@ if ($rev > -1) {
         $ext = 'ico';
     }
     if ($cacheimage) {
-        if (!empty($_SERVER['HTTP_IF_NONE_MATCH'])) {
+        if (!empty($_SERVER['HTTP_IF_NONE_MATCH']) || !empty($_SERVER['HTTP_IF_MODIFIED_SINCE'])) {
             // we do not actually need to verify the etag value because our files
             // never change in cache because we increment the rev parameter
-            header('HTTP/1.1 304 Not Modified');
-
             $lifetime = 60*60*24*30; // 30 days
             $mimetype = get_contenttype_from_ext($ext);
+            header('HTTP/1.1 304 Not Modified');
             header('Expires: '. gmdate('D, d M Y H:i:s', time() + $lifetime) .' GMT');
             header('Cache-Control: max-age='.$lifetime);
             header('Content-Type: '.$mimetype);
diff --git a/theme/javascript.php b/theme/javascript.php
index d5d0d55aa5e..9b359a1f97a 100644
--- a/theme/javascript.php
+++ b/theme/javascript.php
@@ -49,11 +49,11 @@ if (file_exists("$CFG->dirroot/theme/$themename/config.php")) {
 $candidate = "$CFG->dataroot/cache/theme/$themename/javascript_$type.js";
 
 if ($rev > -1 and file_exists($candidate)) {
-    if (!empty($_SERVER['HTTP_IF_NONE_MATCH'])) {
+    if (!empty($_SERVER['HTTP_IF_NONE_MATCH']) || !empty($_SERVER['HTTP_IF_MODIFIED_SINCE'])) {
         // we do not actually need to verify the etag value because our files
         // never change in cache because we increment the rev parameter
-        header('HTTP/1.1 304 Not Modified');
         $lifetime = 60*60*24*30; // 30 days
+        header('HTTP/1.1 304 Not Modified');
         header('Expires: '. gmdate('D, d M Y H:i:s', time() + $lifetime) .' GMT');
         header('Cache-Control: max-age='.$lifetime);
         header('Content-Type: application/javascript; charset=utf-8');
diff --git a/theme/styles.php b/theme/styles.php
index 2a086262772..ce6ada05da9 100644
--- a/theme/styles.php
+++ b/theme/styles.php
@@ -53,11 +53,11 @@ if ($type === 'ie') {
 $candidatesheet = "$CFG->dataroot/cache/theme/$themename/css/$type.css";
 
 if (file_exists($candidatesheet)) {
-    if (!empty($_SERVER['HTTP_IF_NONE_MATCH'])) {
+    if (!empty($_SERVER['HTTP_IF_NONE_MATCH']) || !empty($_SERVER['HTTP_IF_MODIFIED_SINCE'])) {
         // we do not actually need to verify the etag value because our files
         // never change in cache because we increment the rev parameter
-        header('HTTP/1.1 304 Not Modified');
         $lifetime = 60*60*24*30; // 30 days
+        header('HTTP/1.1 304 Not Modified');
         header('Expires: '. gmdate('D, d M Y H:i:s', time() + $lifetime) .' GMT');
         header('Cache-Control: max-age='.$lifetime);
         header('Content-Type: text/css; charset=utf-8');
diff --git a/theme/yui_combo.php b/theme/yui_combo.php
index f3dba345c90..8d46a1d7148 100644
--- a/theme/yui_combo.php
+++ b/theme/yui_combo.php
@@ -49,8 +49,8 @@ if (substr($parts, -3) === '.js') {
 // If-Modified-Since header, we can send back a 304 Not Modified since the
 // content never changes (the rev number is increased any time the content changes)
 if (!empty($_SERVER['HTTP_IF_NONE_MATCH']) || !empty($_SERVER['HTTP_IF_MODIFIED_SINCE'])) {
-    header('HTTP/1.1 304 Not Modified');
     $lifetime = 60*60*24*30; // 30 days
+    header('HTTP/1.1 304 Not Modified');
     header('Expires: '. gmdate('D, d M Y H:i:s', time() + $lifetime) .' GMT');
     header('Cache-Control: max-age='.$lifetime);
     header('Content-Type: '.$mimetype);