diff --git a/src/RocketChatAttachment.php b/src/RocketChatAttachment.php
index 75db712..6db1281 100644
--- a/src/RocketChatAttachment.php
+++ b/src/RocketChatAttachment.php
@@ -4,8 +4,7 @@
 
 namespace NotificationChannels\RocketChat;
 
-use DateTime;
-use DateTimeZone;
+use DateTimeInterface;
 use Illuminate\Support\Str;
 use InvalidArgumentException;
 
@@ -103,18 +102,24 @@ public function text(string $text): self
     }
 
     /**
-     * @param string|\DateTime $timestamp
+     * @param string|\DateTimeInterface $timestamp
      * @return \NotificationChannels\RocketChat\RocketChatAttachment
      */
     public function timestamp($timestamp): self
     {
-        if (! ($timestamp instanceof DateTime) && ! is_string($timestamp)) {
-            throw new InvalidArgumentException('Timestamp must be string or DateTime, '.gettype($timestamp).' given.');
+        if (! ($timestamp instanceof DateTimeInterface) && ! is_string($timestamp)) {
+            $invalidType = is_object($timestamp)
+                ? get_class($timestamp)
+                : gettype($timestamp);
+
+            throw new InvalidArgumentException(sprintf(
+                'Timestamp must be string or DateTime, %s given.',
+                $invalidType
+            ));
         }
 
-        if ($timestamp instanceof DateTime) {
-            $date = clone $timestamp;
-            $timestamp = $date->setTimezone(new DateTimeZone('UTC'))->format('Y-m-d\TH:i:s.v\Z');
+        if ($timestamp instanceof DateTimeInterface) {
+            $timestamp = $timestamp->format(DateTimeInterface::RFC3339);
         }
 
         $this->timestamp = $timestamp;
diff --git a/tests/RocketChatAttachmentTest.php b/tests/RocketChatAttachmentTest.php
index 4dc9b9b..b767fe4 100644
--- a/tests/RocketChatAttachmentTest.php
+++ b/tests/RocketChatAttachmentTest.php
@@ -4,6 +4,10 @@
 
 namespace NotificationChannels\RocketChat\Test;
 
+use DateTime;
+use DateTimeImmutable;
+use DateTimeInterface;
+use InvalidArgumentException;
 use NotificationChannels\RocketChat\RocketChatAttachment;
 use PHPUnit\Framework\TestCase;
 
@@ -63,11 +67,31 @@ public function it_can_set_the_timestamp(): void
     /** @test */
     public function it_can_set_the_timestamp_as_datetime(): void
     {
-        $date = \DateTime::createFromFormat('Y-m-d H:i:s.u', '2020-02-19 19:00:00.000');
+        $date = DateTime::createFromFormat('Y-m-d H:i:s.u', '2020-02-19 19:00:00.000');
         $attachment = new RocketChatAttachment();
         $attachment->timestamp($date);
 
-        $this->assertEquals(['ts' => '2020-02-19T19:00:00.000Z'], $attachment->toArray());
+        $this->assertEquals(['ts' => $date->format(DateTimeInterface::ATOM)], $attachment->toArray());
+    }
+
+    /** @test */
+    public function it_can_set_the_timestamp_as_immutable_datetime(): void
+    {
+        $date = DateTimeImmutable::createFromFormat('Y-m-d H:i:s.u', '2020-02-19 19:00:00.000');
+        $attachment = new RocketChatAttachment();
+        $attachment->timestamp($date);
+
+        $this->assertSame(['ts' => $date->format(DateTimeInterface::ATOM)], $attachment->toArray());
+    }
+
+    /** @test */
+    public function it_cannot_set_the_timestamp_as_integer(): void
+    {
+        $this->expectException(InvalidArgumentException::class);
+
+        $date = 1234567890;
+        $attachment = new RocketChatAttachment();
+        $attachment->timestamp($date);
     }
 
     /** @test */