using System;

using System.Globalization;

public class Program

{

    public static void Main()

    {

        Console.WriteLine(ConvertDateFormat("12/31/2015", "MM/dd/yyyy", "yyyy-MM-dd HH:mm:ss"));

    }

    public static string ConvertDateFormat(string input, string inputFormat, string outputFormat)

        {       

           return DateTime.ParseExact(input, inputFormat, CultureInfo.InvariantCulture)

                          .ToString(outputFormat,CultureInfo.InvariantCulture);

        }

}