using System;

public class Program

{

    public static void Main()

    {

        var someTestFileNames = new []{

            "myFile.txt","myTarGz.tar.gz"

        };

        foreach( var tester in someTestFileNames)

            Console.WriteLine(DatifyFileName(tester));

    }

    /*

    public static string DatifyFileName(string fileName)

    {

        var nameWithoutExtension = System.IO.Path.GetFileNameWithoutExtension(fileName);

        var extension = System.IO.Path.GetExtension(fileName);

        return $"{nameWithoutExtension}_{DateTime.Now.ToUniversalTime().ToString("THHmmssfff")}{extension}";

    }

    */

    public static string DatifyFileName(ReadOnlySpan<char> fileName)

    {

        var lastDotIndex = fileName.LastIndexOf('.');

        //Maybe : if( lastDotIndex < 0 ) throw ArgumentException("no extension found");

        var nameWithoutExtension = fileName[..lastDotIndex];

        var extension = fileName[lastDotIndex..];

        return $"{nameWithoutExtension}_{DateTime.Now.ToUniversalTime().ToString("THHmmssfff")}{extension}";

    }

}