diff --git a/src/Monolog/Processor/PsrLogMessageProcessor.php b/src/Monolog/Processor/PsrLogMessageProcessor.php
index a10cd76c..f845f883 100644
--- a/src/Monolog/Processor/PsrLogMessageProcessor.php
+++ b/src/Monolog/Processor/PsrLogMessageProcessor.php
@@ -20,7 +20,7 @@ namespace Monolog\Processor;
  */
 class PsrLogMessageProcessor
 {
-    const SIMPLE_DATE = "Y-m-d\TH:i:sP";
+    const SIMPLE_DATE = "Y-m-d\TH:i:s.uP";
 
     private $dateFormat;
 
@@ -33,7 +33,7 @@ class PsrLogMessageProcessor
      */
     public function __construct(string $dateFormat = null, bool $removeUsedContextFields = false)
     {
-        $this->dateFormat = null === $dateFormat ? static::SIMPLE_DATE : $dateFormat;
+        $this->dateFormat = $dateFormat;
         $this->removeUsedContextFields = $removeUsedContextFields;
     }
 
@@ -57,9 +57,17 @@ class PsrLogMessageProcessor
             if (is_null($val) || is_scalar($val) || (is_object($val) && method_exists($val, "__toString"))) {
                 $replacements[$placeholder] = $val;
             } elseif ($val instanceof \DateTimeInterface) {
-                $replacements[$placeholder] = $val->format($this->dateFormat);
+                if (!$this->dateFormat && $val instanceof \Monolog\DateTimeImmutable) {
+                    // handle monolog dates using __toString if no specific dateFormat was asked for
+                    // so that it follows the useMicroseconds flag
+                    $replacements[$placeholder] = (string) $val;
+                } else {
+                    $replacements[$placeholder] = $val->format($this->dateFormat ?: static::SIMPLE_DATE);
+                }
             } elseif (is_object($val)) {
                 $replacements[$placeholder] = '[object '.get_class($val).']';
+            } elseif (is_array($val)) {
+                $replacements[$placeholder] = 'array'.@json_encode($val);
             } else {
                 $replacements[$placeholder] = '['.gettype($val).']';
             }
diff --git a/tests/Monolog/Processor/PsrLogMessageProcessorTest.php b/tests/Monolog/Processor/PsrLogMessageProcessorTest.php
index 60fafc64..3b5ec6e9 100644
--- a/tests/Monolog/Processor/PsrLogMessageProcessorTest.php
+++ b/tests/Monolog/Processor/PsrLogMessageProcessorTest.php
@@ -68,7 +68,11 @@ class PsrLogMessageProcessorTest extends \PHPUnit\Framework\TestCase
             [false,    ''],
             [$date, $date->format(PsrLogMessageProcessor::SIMPLE_DATE)],
             [new \stdClass, '[object stdClass]'],
-            [[], '[array]'],
+            [[], 'array[]'],
+            [[], 'array[]'],
+            [[1, 2, 3], 'array[1,2,3]'],
+            [['foo' => 'bar'], 'array{"foo":"bar"}'],
+            [stream_context_create(), '[resource]'],
         ];
     }
 }