diff --git a/CHANGELOG.md b/CHANGELOG.md
index e533194e..734ddbba 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -14,6 +14,7 @@ For a full diff see [`7afa59c...main`][7afa59c...main].
 * Added `SlowTestCollector` ([#8]), by [@localheinz]
 * Added `Subscriber\TestPreparedSubscriber` ([#12]), by [@localheinz]
 * Added `Subscriber\TestPassedSubscriber` ([#13]), by [@localheinz]
+* Added `Formatter\ToMillisecondsDurationFormatter` ([#17]), by [@localheinz]
 
 [7afa59c...main]: https://github.com/ergebnis/phpunit-slow-test-detector/compare/7afa59c...main
 
@@ -21,5 +22,6 @@ For a full diff see [`7afa59c...main`][7afa59c...main].
 [#8]: https://github.com/ergebnis/phpunit-slow-test-detector/pull/8
 [#12]: https://github.com/ergebnis/phpunit-slow-test-detector/pull/12
 [#13]: https://github.com/ergebnis/phpunit-slow-test-detector/pull/13
+[#17]: https://github.com/ergebnis/phpunit-slow-test-detector/pull/137
 
 [@localheinz]: https://github.com/localheinz
diff --git a/src/Formatter/ToMillisecondsDurationFormatter.php b/src/Formatter/ToMillisecondsDurationFormatter.php
new file mode 100644
index 00000000..0940d5c4
--- /dev/null
+++ b/src/Formatter/ToMillisecondsDurationFormatter.php
@@ -0,0 +1,35 @@
+<?php
+
+declare(strict_types=1);
+
+/**
+ * Copyright (c) 2021 Andreas Möller
+ *
+ * For the full copyright and license information, please view
+ * the LICENSE.md file that was distributed with this source code.
+ *
+ * @see https://github.com/ergebnis/phpunit-slow-test-detector
+ */
+
+namespace Ergebnis\PHPUnit\SlowTestDetector\Formatter;
+
+use PHPUnit\Event;
+
+final class ToMillisecondsDurationFormatter implements Event\Telemetry\DurationFormatter
+{
+    public function format(Event\Telemetry\Duration $duration): string
+    {
+        $milliseconds = self::toMilliseconds($duration);
+
+        return \sprintf(
+            '%s ms',
+            \number_format($milliseconds)
+        );
+    }
+
+    private static function toMilliseconds(Event\Telemetry\Duration $duration): int
+    {
+        return $duration->seconds() * (10 ** 3)
+            + (int) \round($duration->nanoseconds() / (10 ** 6));
+    }
+}
diff --git a/test/Unit/Formatter/ToMillisecondsDurationFormatterTest.php b/test/Unit/Formatter/ToMillisecondsDurationFormatterTest.php
new file mode 100644
index 00000000..72f28eea
--- /dev/null
+++ b/test/Unit/Formatter/ToMillisecondsDurationFormatterTest.php
@@ -0,0 +1,97 @@
+<?php
+
+declare(strict_types=1);
+
+/**
+ * Copyright (c) 2021 Andreas Möller
+ *
+ * For the full copyright and license information, please view
+ * the LICENSE.md file that was distributed with this source code.
+ *
+ * @see https://github.com/ergebnis/phpunit-slow-test-detector
+ */
+
+namespace Ergebnis\PHPUnit\SlowTestDetector\Test\Unit\Formatter;
+
+use Ergebnis\PHPUnit\SlowTestDetector\Formatter\ToMillisecondsDurationFormatter;
+use Ergebnis\Test\Util;
+use PHPUnit\Event;
+use PHPUnit\Framework;
+
+/**
+ * @internal
+ *
+ * @covers \Ergebnis\PHPUnit\SlowTestDetector\Formatter\ToMillisecondsDurationFormatter
+ */
+final class ToMillisecondsDurationFormatterTest extends Framework\TestCase
+{
+    use Util\Helper;
+
+    /**
+     * @dataProvider provideDurationAndFormattedDuration
+     */
+    public function testFormatFormats(Event\Telemetry\Duration $duration, string $formattedDuration): void
+    {
+        $formatter = new ToMillisecondsDurationFormatter();
+
+        self::assertSame($formattedDuration, $formatter->format($duration));
+    }
+
+    /**
+     * @return array<string, array{0: Event\Telemetry\Duration, 1: string}>
+     */
+    public function provideDurationAndFormattedDuration(): array
+    {
+        return [
+            'zero' => [
+                Event\Telemetry\Duration::fromSecondsAndNanoseconds(
+                    0,
+                    0
+                ),
+                '0 ms',
+            ],
+            'nanoseconds-rounded-down' => [
+                Event\Telemetry\Duration::fromSecondsAndNanoseconds(
+                    0,
+                    499_999
+                ),
+                '0 ms',
+            ],
+            'nanoseconds-rounded-up' => [
+                Event\Telemetry\Duration::fromSecondsAndNanoseconds(
+                    0,
+                    500_000
+                ),
+                '1 ms',
+            ],
+            'milliseconds-one' => [
+                Event\Telemetry\Duration::fromSecondsAndNanoseconds(
+                    0,
+                    1_000_000
+                ),
+                '1 ms',
+            ],
+            'milliseconds-hundreds' => [
+                Event\Telemetry\Duration::fromSecondsAndNanoseconds(
+                    0,
+                    123 * 1_000_000
+                ),
+                '123 ms',
+            ],
+            'seconds' => [
+                Event\Telemetry\Duration::fromSecondsAndNanoseconds(
+                    1,
+                    1_000_000
+                ),
+                '1,001 ms',
+            ],
+            'thousands-of-seconds' => [
+                Event\Telemetry\Duration::fromSecondsAndNanoseconds(
+                    1_234,
+                    567_890_123
+                ),
+                '1,234,568 ms',
+            ],
+        ];
+    }
+}