using System;

using System.Linq;

using System.Globalization;

​

public class Program

{

    public static void Main()

    {

        string input = "15th Oct 2018";

        DateTime result = DateTime.Now;

        string[] patterns = {"d'st' MMM yyyy", "d'nd' MMM yyyy", "d'rd' MMM yyyy", "d'th' MMM yyyy"};

        bool success = DateTime.TryParseExact(input, patterns, CultureInfo.InvariantCulture, DateTimeStyles.None, out result);

        if (success)

            Console.WriteLine(result.ToString("yyyy-MM-dd"));

    }

}