using System;

public class Program

{

    const string dateToParse = "Sunday, 22 September 2019";

    const string dateFormat = "dddd, dd MMMM yyyy";

    const string dateOutFormat = "yyyy-MM-dd";

    public static void Main()

    {

        var date = DateTime.ParseExact(dateToParse, dateFormat, System.Globalization.CultureInfo.InvariantCulture);

        Console.WriteLine(date.ToString(dateOutFormat, System.Globalization.CultureInfo.InvariantCulture));

    }

}