using System;

using System.IO;

public class Program

{

    private static string GenerateLogFileName(string filePathBase)

        {

            var ext= Path.GetExtension(filePathBase);

            var fn= Path.GetFileNameWithoutExtension(filePathBase);

            var dateStamp = DateTime.UtcNow.ToString("yyyy-MM-dd");

            var newFileName = string.Format("{0}-{1}{2}", fn, dateStamp, ext);

            return Path.Combine(Path.GetDirectoryName(filePathBase), newFileName);

        }

​

    public static void Main()

    {

        Console.WriteLine(GenerateLogFileName("logs/log.txt"));

        Console.WriteLine(GenerateLogFileName(@"c:\windows\logs\cbs\cbspersist.log"));

    }

}