using System;

​

public class Program

{

    public static void Main()

    {

        var time = "17/8/2015 11:43:48 AM";

        Console.WriteLine("Before: " + time);

        DateTime newDT = DateTime.ParseExact(time, "dd/M/yyyy hh:mm:ss tt", null);

        Console.WriteLine("After: " + newDT.ToString("yyyy-MM-dd HH:mm:ss.fff"));

    }

}