First of all you XML is not a valid XML file.
It should look like this
="1.0"
<catalog>
<book id="bk101">
<author>Gambardella, Matthew</author>
<title>XML Developer's Guide</title>
</book>
<created>
<createddate>
<DateTime>
2015-05-16T05:50:06.7199222-04:00
</DateTime>
</createddate>
</created>
</catalog>
An open tag must have a close tag.
e.g.
<created></created>
Moreover your SelectSingleNode call does not respect capitalisation.
It must rather be like this:
xmlDoc.SelectSingleNode("catalog/created/createddate/DateTime").InnerText = DateTime.Now.ToString("dd.MM.yyyy");
InnerText is of type string.
Therefore you must convert your DateTime object to a string as it is shown in the example above.