Default to microtime() instead of DateTimeImmutable

src/Generator/UnixTimeGenerator.php

@@ -15,7 +15,6 @@ declare(strict_types=1);
 namespace Ramsey\Uuid\Generator;
 
 use Brick\Math\BigInteger;
-use DateTimeImmutable;
 use DateTimeInterface;
 use Ramsey\Uuid\Type\Hexadecimal;
 
@@ -68,7 +67,12 @@ class UnixTimeGenerator implements TimeGeneratorInterface
      */
     public function generate($node = null, ?int $clockSeq = null, ?DateTimeInterface $dateTime = null): string
     {
-        $time = ($dateTime ?? new DateTimeImmutable('now'))->format('Uv');
+        if ($dateTime === null) {
+            $time = microtime(false);
+            $time = substr($time, 11) . substr($time, 2, 3);
+        } else {
+            $time = $dateTime->format('Uv');
+        }
 
         if ($time > self::$time || ($dateTime !== null && $time !== self::$time)) {
             $this->randomize($time);