using System;

public class Program

{

    public const string DATETIME_INPUT = @"

2017/07/13 12:08:09.999 AM

2017/07/13 01:08:09.999 AM

2017/07/13 02:08:09.999 AM

2017/07/13 03:08:09.999 AM

2017/07/13 04:08:09.999 AM

2017/07/13 05:08:09.999 AM

2017/07/13 06:08:09.999 AM

2017/07/13 07:08:09.999 AM

2017/07/13 08:08:09.999 AM

2017/07/13 09:08:09.999 AM

2017/07/13 10:08:09.999 AM

2017/07/13 11:08:09.999 AM

2017/07/13 12:08:09.999 PM

2017/07/13 01:08:09.999 PM

2017/07/13 02:08:09.999 PM

2017/07/13 03:08:09.999 PM

2017/07/13 04:08:09.999 PM

2017/07/13 05:08:09.999 PM

2017/07/13 06:08:09.999 PM

2017/07/13 07:08:09.999 PM

2017/07/13 08:08:09.999 PM

2017/07/13 09:08:09.999 PM

2017/07/13 10:08:09.999 PM

2017/07/13 11:08:09.999 PM

";

​

    public static void Main()

    {

        processdata();

    }

    public static void processdata()

    {

        string[] datalines = DATETIME_INPUT.Split('\n');

        for (var i=0; i < datalines.Length; i++) 

        {

            if (datalines[i] != "")

            {

                Console.WriteLine(processrow(datalines[i]));

            }

        }

    }

    public static string processrow(string row)

    {

        System.DateTime datetime1;

        if (System.DateTime.TryParse(row, out datetime1))

        {

            return datetime1.ToString("yyyy-MM-dd HH:mm:ss.fff");

        }

        return "Invalid Date Format";

    }

}

​