Hi,
Whenever we are trying set the one value to node it should be format of string only. Before you are setting the value, you have to convert it into string then try to set this value.
Date = DateTime.Now.ToString("yyyy-MM-ddTHH:mm:ss");
thisxdocument.SelectSingleNode(
"Datenodexpath", Date).text;
Datenodexpath --> Enter your date field xpath