Replace DateT DateTimeImmutable

Author: sylfabre

src/AbsoluteDate.php

@@ -180,7 +180,7 @@ public function __toString(): string
      */
     public static function createInTimezone(\DateTimeZone $timezone, \DateTimeInterface $datetime = null): self
     {
-        $datetime = new \DateTime('@' . (null === $datetime ? time() : $datetime->getTimestamp()));
+        $datetime = new \DateTimeImmutable('@' . (null === $datetime ? time() : $datetime->getTimestamp()));
         $datetime->setTimezone($timezone);
 
         return new self($datetime->format(self::DEFAULT_DATE_FORMAT));
@@ -198,7 +198,7 @@ public static function createRelative(string $relative = 'now', \DateTimeZone $t
             $timezone = new \DateTimeZone('UTC');
         }
 
-        $datetime = new \DateTime($relative, $timezone);
+        $datetime = new \DateTimeImmutable($relative, $timezone);
 
         return self::createInTimezone($timezone, $datetime);
     }