string textFile = System.IO.File.ReadAllText(@"C:\Users\Александр\Desktop\good.htm");
textFile = textFile.Replace("datatimes", this.clientsItem.dateconnect.ToString());
File.AppendAllText(Environment.GetFolderPath(Environment.SpecialFolder.MyDocuments) + "\\otchet.htm", textFile, System.Text.Encoding.GetEncoding("utf-8"));
Regex.Replace(textFile, "datatimes", this.clientsItem.dateconnect.ToString(), RegexOptions.IgnoreCase)