Demo

CSharp · Console

/Program.cs

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"));
	}
}