title: Change format of DateTime
body: <p>I would like to change the format of datetime but I get error:</p>
  
  <blockquote>
    <p>No overload for method 'ToString' takes 1 arguments</p>
  </blockquote>
  
  <pre><code>string ret = person.ArkivDate.ToString("yyyy-MM-dd")
  </code></pre>
  
  <p>The type of ArkivDate is <code>DateTime?</code></p>
  
  <p>How can I fix it? </p>