using System;

using System.Collections;

using System.Linq;

​

​

public class Program

{

    public static void Main()

    {

        ArrayList list = new ArrayList() {

            "1;06/07/2017 11:17:16;out;0",

            "1;06/07/2017 11:27:16;out;2",

            "1;06/07/2017 11:28:16;out;0",

            "1;06/07/2017 11:20:16;out;3"

        };

        var distinctEnumerable = list.Cast<string>()

            .GroupBy(x => {

                var items = x.Split(';');

                var dateParts = items[1].Split(' ');

                return string.Join(";", items[0], dateParts[0], items[2]);

            })

            .Select(x => x.Key).ToList();

        Console.WriteLine(string.Join("\n", distinctEnumerable));

    }

}