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