Hi,
This code is working just fine for me:
// Create an instance of the Tree provider first
TreeProvider tree = new TreeProvider(CMSContext.CurrentUser);
// Get the document
TreeNode node = tree.SelectSingleNode(CMSContext.CurrentSiteName, "/home", "en-us");
DateTime date = DateTime.Now;
node.SetValue("DocumentModifiedWhen", date);
node.Update();
What is your code like?
Best regards,
Juraj Ondrus