DateTime.ToString perf test when format string is fixed

Program.cs

using System;
using System.Diagnostics;
using System.Text;
using System.Threading;

namespace DateTimeFormatPerfTest
{
    public class Program
    {
        private static object _locker = new object();
        private static DateTime _now = DateTime.Now;
        private static Timer _timer = new Timer(UpdateNow, null, 1000, 1000);

        private static void UpdateNow(object state)
        {
            lock (_locker)
            {
                _now = DateTime.Now;
            }
        }

        public void Main(string[] args)
        {
            var iterations = 5000000;
            var sw = new Stopwatch();

            sw.Start();
            for (int i = 0; i < iterations; i++)
            {
                DateTimeToString(_now);   
            }
            sw.Stop();
            Console.WriteLine($"{nameof(DateTimeToString)} ({Encoding.UTF8.GetString(DateTimeToString(_now))}) completed {iterations} iterations in {sw.Elapsed}");

            sw.Restart();
            for (int i = 0; i < iterations; i++)
            {
                DateTimeToStringFast(_now);
            }
            sw.Stop();
            Console.WriteLine($"{nameof(DateTimeToStringFast)} ({Encoding.UTF8.GetString(DateTimeToStringFast(_now))}) completed {iterations} iterations in {sw.Elapsed}");

            _timer.Dispose();
        }

        private static string _format = "yyyyMMddTHHmmssZ";

        private byte[] DateTimeToString(DateTime dateTime)
        {
            string dateTimeString;
            lock (_locker)
            {
                dateTimeString = dateTime.ToString(_format);
            }
            return Encoding.UTF8.GetBytes(dateTimeString);
        }
        
        private static byte[] _numbers = new[] {
            Convert.ToByte('0'),
            Convert.ToByte('1'),
            Convert.ToByte('2'),
            Convert.ToByte('3'),
            Convert.ToByte('4'),
            Convert.ToByte('5'),
            Convert.ToByte('6'),
            Convert.ToByte('7'),
            Convert.ToByte('8'),
            Convert.ToByte('9'),
        };

        private static byte[] _specialChars = new byte[] { Convert.ToByte('T'), Convert.ToByte('Z') };
        
        private byte[] DateTimeToStringFast(DateTime dateTime)
        {
            var result = new byte[_format.Length];

            int yearInt;
            int monthInt;
            int dayInt;
            int hourInt;
            int minuteInt;
            int secondInt;

            lock (_locker)
            {
                yearInt = dateTime.Year;
                monthInt = dateTime.Month;
                dayInt = dateTime.Day;
                hourInt = dateTime.Hour;
                minuteInt = dateTime.Minute;
                secondInt = dateTime.Second;
            }

            // Year
            var yearThousands = (int)Math.Floor(yearInt / 1000d) * 1000;
            var yearHundreds = (int)Math.Floor((yearInt - yearThousands) / 100d) * 100;
            var yearTens = (int)Math.Floor((yearInt - yearThousands - yearHundreds) / 10d) * 10;
            var yearSingles = yearInt - yearThousands - yearHundreds - yearTens;
            result[0] = _numbers[yearThousands / 1000];
            result[1] = _numbers[yearHundreds / 100];
            result[2] = _numbers[yearTens / 10];
            result[3] = _numbers[yearSingles];

            // Month
            var monthTens = (int)Math.Floor(monthInt / 10d) * 10;
            var monthSingles = monthInt - monthTens;
            result[4] = _numbers[monthTens / 10];
            result[5] = _numbers[monthSingles];

            // Day
            var dayTens = (int)Math.Floor(dayInt / 10d) * 10;
            var daySingles = dayInt - dayTens;
            result[6] = _numbers[dayTens / 10];
            result[7] = _numbers[daySingles];

            // T
            result[8] = _specialChars[0];

            // Hour
            var hourTens = (int)Math.Floor(hourInt / 10d) * 10;
            var hourSingles = hourInt - hourTens;
            result[9] = _numbers[hourTens / 10];
            result[10] = _numbers[hourSingles];

            // Minute
            var minuteTens = (int)Math.Floor(minuteInt / 10d) * 10;
            var minuteSingles = minuteInt - minuteTens;
            result[11] = _numbers[minuteTens / 10];
            result[12] = _numbers[minuteSingles];

            // Second
            var secondTens = (int)Math.Floor(secondInt / 10d) * 10;
            var secondSingles = secondInt - secondTens;
            result[13] = _numbers[secondTens / 10];
            result[14] = _numbers[secondSingles];

            // Z
            result[15] = _specialChars[1];

            return result;
        }
    }
}

Results.txt

DateTimeToString (20150801T225229Z) completed 5000000 iterations in 00:00:04.8045709
DateTimeToStringFast (20150801T225231Z) completed 5000000 iterations in 00:00:01.8000201