diff --git a/controller/Controller.php b/controller/Controller.php
index 115251dae..ca50c0959 100644
--- a/controller/Controller.php
+++ b/controller/Controller.php
@@ -293,7 +293,7 @@ protected function sendNotModifiedHeader($modifiedDate): bool
     {
         if ($modifiedDate) {
             $ifModifiedSince = $this->getIfModifiedSince();
-            $this->sendHeader("Last-Modified: " . $modifiedDate->format('Y-m-d H:i:s'));
+            $this->sendHeader("Last-Modified: " . $modifiedDate->format('D, d M Y H:i:s \G\M\T'));
             if ($ifModifiedSince !== null && $ifModifiedSince >= $modifiedDate) {
                 $this->sendHeader("HTTP/1.0 304 Not Modified");
                 return true;
@@ -308,9 +308,10 @@ protected function sendNotModifiedHeader($modifiedDate): bool
     protected function getIfModifiedSince()
     {
         $ifModifiedSince = null;
-        if (isset($_SERVER["HTTP_IF_MODIFIED_SINCE"])) {
-            // example value set by a browser: "2019-04-13 08:28:23"
-            $ifModifiedSince = DateTime::createFromFormat("Y-m-d H:i:s", $_SERVER["HTTP_IF_MODIFIED_SINCE"]);
+        $ifModSinceHeader = filter_input(INPUT_SERVER, 'HTTP_IF_MODIFIED_SINCE', FILTER_SANITIZE_STRING);
+        if ($ifModSinceHeader) {
+            // example value set by a browser: "Mon, 11 May 2020 10:46:57 GMT"
+            $ifModifiedSince = new DateTime($ifModSinceHeader);
         }
         return $ifModifiedSince;
     }
diff --git a/tests/Http304Test.php b/tests/Http304Test.php
index 6eb6eb590..f3d683b59 100644
--- a/tests/Http304Test.php
+++ b/tests/Http304Test.php
@@ -149,10 +149,12 @@ public function testHttp304FirstEverRequest()
             $modifiedDate = DateTime::createFromFormat('j-M-Y', '15-Feb-2009');
             $this->controller
                 ->shouldReceive("getModifiedDate")
+                ->once()
                 ->andReturn($modifiedDate);
             $this->controller
                 ->shouldReceive("sendHeader")
-                ->withArgs(["Last-Modified: " . $modifiedDate->format('Y-m-d H:i:s')])
+                ->once()
+                ->withArgs(["Last-Modified: " . $modifiedDate->format('D, d M Y H:i:s \G\M\T')])
                 ->andReturn(true);
         }
 
@@ -204,16 +206,20 @@ public function testHttp304()
             $ifModifiedSince = DateTime::createFromFormat('j-M-Y', '15-Feb-2019');
             $this->controller
                 ->shouldReceive("getModifiedDate")
+                ->once()
                 ->andReturn($modifiedDate);
             $this->controller
                 ->shouldReceive("getIfModifiedSince")
+                ->once()
                 ->andReturn($ifModifiedSince);
             $this->controller
                 ->shouldReceive("sendHeader")
-                ->withArgs(["Last-Modified: " . $modifiedDate->format('Y-m-d H:i:s')])
+                ->once()
+                ->withArgs(["Last-Modified: " . $modifiedDate->format('D, d M Y H:i:s \G\M\T')])
                 ->andReturn(true);
             $this->controller
                 ->shouldReceive("sendHeader")
+                ->once()
                 ->withArgs(["HTTP/1.0 304 Not Modified"])
                 ->andReturn(true);
         }
@@ -223,4 +229,10 @@ public function testHttp304()
         $content = ob_get_clean();
         $this->assertEquals("", $content);
     }
+
+    public function tearDown()
+    {
+        parent::tearDown();
+        \Mockery::close();
+    }
 }