using System;
using System.Linq;
using System.Collections.Generic;
using System.Globalization;

public class Test
{
	public static void Main()
	{
	    string[] dateStrings = new[] { "Friday 31 August 2012", "26-Jul-2012", "190811"};
        DateTime date = DateTime.MinValue;
        string[] formats = new[] { "dddd dd MMMM yyyy", "dd-MMM-yyyy", "ddMMyy" };
        IEnumerable<DateTime> dates = dateStrings
        .Where(ds => DateTime.TryParseExact(ds
            , formats
            , CultureInfo.InvariantCulture
            , DateTimeStyles.None
            , out date))
        .Select(ds => date);
        foreach(DateTime d in dates)
            Console.WriteLine(d.ToString());
	}
}