// Online C# Editor for free

// Write, Edit and Run your C# code using C# Online Compiler

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using System;

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public class HelloWorld

{

    public static void Main(string[] args)

    {

        string filename="f6600af200.ext.230106.2";

        string dateFromFilename = filename.Split('.')[2];

        string asOfDate = DateTime.ParseExact(dateFromFilename, "yyMMdd", System.Globalization.CultureInfo.InvariantCulture).ToString("yyyy-MM-dd HH:mm:ss.fff");

        Console.WriteLine(dateFromFilename);

        Console.WriteLine(asOfDate,"\n");

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    }

}