test.cs

using System;
using System.Globalization;
using System.IO;
using System.Linq;
using CsvHelper;

namespace ConsoleApp1
{
  class Program
  {
    enum CommandType { Get, Post, };

    static void Main(string[] args)
    {
      var datum = DateTime.ParseExact(args[0], "yyyy-MM-dd", CultureInfo.InvariantCulture);
      var datumString = datum.ToString("dd/MMM/yyyy", CultureInfo.InvariantCulture);

      using (var reader = new StreamReader(@"D:\JAGR\access.log"))
      using (var csv = new CsvReader(reader))
      {
        csv.Configuration.BadDataFound = null;
        csv.Configuration.HasHeaderRecord = false;
        csv.Configuration.Delimiter = " ";
        var records = csv.GetRecords<dynamic>().Where(x => ((String)x.Field4).IndexOf(datumString) >= 0).ToArray();

        var data = records.Select(x => new
        {
          Date = DateTime.ParseExact(new String(((String)x.Field4).Skip(1).Take(11).ToArray()), "dd/MMM/yyyy", CultureInfo.InvariantCulture),
          Command = ((String)x.Field6).StartsWith("GET") ? CommandType.Get : CommandType.Post,
          Agent = new String(((String)x.Field10).TakeWhile(c => c != ' ').ToArray()),
        })
        .Where(x => x.Date == datum)
        .ToArray();

        Console.WriteLine($"Number of GETs = {data.Where(x => x.Command == CommandType.Get).Count()}.");
        Console.WriteLine($"Number of POSTs= {data.Where(x => x.Command == CommandType.Post).Count()}.");
        Console.WriteLine($"Number of distinct UA = {data.Select(x => x.Agent).Distinct().Count()}.");
        Console.ReadLine();
      }
    }
  }
}