diff --git a/src/Monolog/Handler/MandrillHandler.php b/src/Monolog/Handler/MandrillHandler.php
index 3f0956a9..de039a86 100644
--- a/src/Monolog/Handler/MandrillHandler.php
+++ b/src/Monolog/Handler/MandrillHandler.php
@@ -50,7 +50,11 @@ class MandrillHandler extends MailHandler
     {
         $message = clone $this->message;
         $message->setBody($content);
-        $message->setDate(time());
+        if (version_compare(\Swift::VERSION, '6.0.0', '>=')) {
+            $message->setDate(new \DateTimeImmutable());
+        } else {
+            $message->setDate(time());
+        }
 
         $ch = curl_init();