title: DateTimeImmutable vs DateTime
body: <p>The 2 classes <code>DateTime</code> and <code>DateTimeImmutable</code> implement the same interface <code>DateTimeInterface</code>. Therefore I want to know :</p>
  <p><strong>What is the difference</strong> between these 2 classes <code>DateTime</code> and <code>DateTimeImmutable</code>?</p>