How to put time stamp as a file name to a output txt file in C# script

Use DateTime.Now to get the current timestamp, then call .ToString on it with the date format you want (for example, yyyy-MM-dd-HH-mm-ss which is year-month-day-hour-minute-second) and put that in your file name. Also, if you use a verbatim string literal (with @), you don't have to escape the backslashes.

System.IO.File.AppendAllText(string.Format(@"C:\izaz\{0}.txt", DateTime.Now.ToString("yyyy-MM-dd-HH-mm-ss")), "text to append");

You can try

System.IO.File.AppendAllText(@"C:\\izaz\\"+DateTime.Now.ToString("dd_MM_yyyy_hh_mm_ss")+".txt", Envireonment.NewLine + "Duration: " + s + Environment.NewLine);

You can do like this-

string customFileName=@"C:\\izaz\\"+DateTime.Now.ToString("yyyyMMddHHmmssffff")+".txt";
System.IO.File.AppendAllText(customFileName,Envireonment.NewLine+"Duration: "+s+Environment.NewLine);

Hope, it helps :)