using System;

using System.Globalization;

public class Program

{

    public static void Main()

    {

        String txtDate = "14-05-2015 10:42";

        String timeStamp = null;

        DateTime dateValue;

        if (DateTime.TryParse(txtDate, CultureInfo.CreateSpecificCulture("fr-FR"), DateTimeStyles.None, out dateValue))

        {

            timeStamp = GetTimestamp(dateValue);

        }

        Console.WriteLine(timeStamp);

    }

    public static String GetTimestamp(DateTime val)

    {

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

    }

}