using System;

using System.Globalization;

// see: https://stackoverflow.com/q/5366285/1016343

public class Program

{

    public static void Main()

    {

        var dtStr = "11.12.2017 08:39:39";

        bool success = DateTime.TryParseExact(dtStr, "dd.MM.yyyy hh:mm:ss",

                    CultureInfo.InvariantCulture, DateTimeStyles.None, out DateTime changeDate);

             if (!success)

                DateTime.TryParse(dtStr, out  changeDate);

        Console.WriteLine($"{success}: {changeDate}");

​

    }

}

​

// using System.Globalization;

public static class Extensions

{

    public static DateTime? toDate(this string dateTimeStr, string[] dateFmt)

    {

        // example: var dt = "2011-03-21 13:26".toDate(new string[]{"yyyy-MM-dd HH:mm", 

        //                                                  "M/d/yyyy h:mm:ss tt"});

        const DateTimeStyles style = DateTimeStyles.AllowWhiteSpaces;

        if (dateFmt == null)

        {

            var dateInfo = System.Threading.Thread.CurrentThread.CurrentCulture.DateTimeFormat;

            dateFmt=dateInfo.GetAllDateTimePatterns();

        }

        DateTime? result = null;

        DateTime dt;

        if (DateTime.TryParseExact(dateTimeStr, dateFmt,

            CultureInfo.InvariantCulture, style, out dt)) result = dt;

        return result;

    }

    public static DateTime? toDate(this string dateTimeStr, string dateFmt=null)

    {

        // example: var dt="2011-03-21 13:26".toDate("yyyy-MM-dd HH:mm");

        // call overloaded function with string array param

        string[] dateFmtArr = dateFmt==null ? null : new string[] { dateFmt };

        return toDate(dateTimeStr, dateFmtArr);

    }

}