diff --git a/src/Monolog/Logger.php b/src/Monolog/Logger.php
index ad7da3bf..91a6ca4c 100644
--- a/src/Monolog/Logger.php
+++ b/src/Monolog/Logger.php
@@ -168,7 +168,7 @@ class Logger
             'level' => $level,
             'level_name' => self::getLevelName($level),
             'channel' => $this->name,
-            'datetime' => \DateTime::createFromFormat('U.u', sprintf('%.6f', microtime(true))),
+            'datetime' => \DateTime::createFromFormat('U.u', sprintf('%.6F', microtime(true))),
             'extra' => array(),
         );
         // check if any message will handle this message
diff --git a/tests/Monolog/TestCase.php b/tests/Monolog/TestCase.php
index 8624d071..1067b919 100644
--- a/tests/Monolog/TestCase.php
+++ b/tests/Monolog/TestCase.php
@@ -24,7 +24,7 @@ class TestCase extends \PHPUnit_Framework_TestCase
             'level' => $level,
             'level_name' => Logger::getLevelName($level),
             'channel' => 'test',
-            'datetime' => new \DateTime(),
+            'datetime' => \DateTime::createFromFormat('U.u', sprintf('%.6F', microtime(true))),
             'extra' => array(),
         );
     }