results.md

                               Method |       Mean |     Error |     StdDev |     Median |  Gen 0 | Allocated |
------------------------------------- |-----------:|----------:|-----------:|-----------:|-------:|----------:|
                Lyrcaxis_OriginalCode | 2,553.6 ns | 49.699 ns |  53.177 ns | 2,548.6 ns | 0.1526 |     891 B |
                        StringBuilder | 1,908.8 ns | 31.472 ns |  24.571 ns | 1,896.5 ns | 0.0687 |     417 B |
                         StringConcat | 1,990.0 ns | 38.910 ns |  36.396 ns | 1,980.9 ns | 0.1068 |     626 B |
                            NoPadLeft | 2,213.3 ns | 78.367 ns | 231.066 ns | 2,107.1 ns | 0.0992 |     578 B |
              StringBuilder_NoPadLeft | 1,891.5 ns | 37.899 ns | 105.646 ns | 1,857.5 ns | 0.0553 |     321 B |
                                 Math | 1,887.0 ns | 43.834 ns | 129.244 ns | 1,844.5 ns | 0.1163 |     674 B |
                   StringBuilder_Math | 1,577.2 ns | 31.641 ns |  40.016 ns | 1,571.8 ns | 0.0381 |     225 B |
               Math_And_Prebuilt_Days |   448.1 ns | 14.374 ns |  41.931 ns |   438.2 ns | 0.0973 |     562 B |
 StringBuilder_Math_And_Prebuilt_Days |   150.8 ns |  3.137 ns |   8.154 ns |   149.3 ns | 0.0179 |     104 B |

TimeFormatting.cs

using System;
using System.Text;
using BenchmarkDotNet.Attributes;

namespace BenchmarkFull
{
    // @Lyrcaxis on the C# discord asked for help improving the string interpolation code below, and someone dared suggest that string.Concat wasn't good
    // and I couldn't let that rest!

    [MemoryDiagnoser]
    public class TimeFormatting
    {
        // credit to @sc_holden for thinking of doing this
        private readonly string[] PrebuiltDaysString = new string[] {
            ") " + DayOfWeek.Sunday+ "  ",
            ") " + DayOfWeek.Monday + "  ",
            ") " + DayOfWeek.Tuesday + "  ",
            ") " + DayOfWeek.Wednesday+ "  ",
            ") " + DayOfWeek.Thursday+ "  ",
            ") " + DayOfWeek.Friday+ "  ",
            ") " + DayOfWeek.Saturday+ "  "
        };

        private DayOfWeek Day = DateTime.Now.DayOfWeek;
        private int Week = 12; // actually week number
        private int Hour = DateTime.Now.Hour;
        private int Minute = DateTime.Now.Minute;
        private int Second = DateTime.Now.Second;

        private StringBuilder builder = new StringBuilder();

        [Benchmark]
        public string Lyrcaxis_OriginalCode()
        {
            return $"({(int)Day + (Week * 7)}) {Day}  {Hour:00}:{Minute:00}:{Second:00}";
        }

        [Benchmark]
        public string StringBuilder()
        {
            builder.Clear();
            builder.Append("(");
            builder.Append((int)Day + (Week * 7));
            builder.Append(") ");
            builder.Append(Day);
            builder.Append("  ");
            builder.Append(Hour.ToString().PadLeft(2, '0'));
            builder.Append(":");
            builder.Append(Minute.ToString().PadLeft(2, '0'));
            builder.Append(":");
            builder.Append(Second.ToString().PadLeft(2, '0'));

            return builder.ToString();
        }

        [Benchmark]
        public string StringConcat()
        {
            return "(" +
                ((int)Day + (Week * 7)) +
                ") " +
                Day +
                "  " +
                Hour.ToString().PadLeft(2, '0') +
                ":" +
                Minute.ToString().PadLeft(2, '0') +
                ":" +
                Second.ToString().PadLeft(2, '0');
        }

        [Benchmark]
        public string NoPadLeft()
        {
            return "(" +
                ((int)Day + (Week * 7)) +
                ") " +
                Day +
                "  " +
                (Hour < 10 ? "0" : "") + Hour +
                ":" +
                (Minute < 10 ? "0" : "") + Minute +
                ":" +
                (Second < 10 ? "0" : "") + Second;
        }

        [Benchmark]
        public string StringBuilder_NoPadLeft()
        {
            builder.Clear();
            builder.Append("(");
            builder.Append((int)Day + (Week * 7));
            builder.Append(") ");
            builder.Append(Day);
            builder.Append("  ");
            if (Hour < 10)
            {
                builder.Append("0");
            }
            builder.Append(Hour);
            builder.Append(":");
            if (Minute < 10)
            {
                builder.Append("0");
            }
            builder.Append(Minute);
            builder.Append(":");
            if (Second < 10)
            {
                builder.Append("0");
            }
            builder.Append(Second);

            return builder.ToString();
        }


        [Benchmark]
        public string Math()
        {
            return "(" +
                ((int)Day + (Week * 7)) +
                ") " +
                Day +
                "  " +
                (char)((Hour / 10) + 48) +
                (char)((Hour % 10) + 48) +
                ":" +
                (char)((Minute / 10) + 48) +
                (char)((Minute % 10) + 48) +
                ":" +
                (char)((Second / 10) + 48) +
                (char)((Second % 10) + 48);
        }

        [Benchmark]
        public string StringBuilder_Math()
        {
            builder.Clear();
            builder.Append("(");
            builder.Append((int)Day + (Week * 7));
            builder.Append(") ");
            builder.Append(Day);
            builder.Append("  ");
            builder.Append((char)((Hour / 10) + 48));
            builder.Append((char)((Hour % 10) + 48));
            builder.Append(":");
            builder.Append((char)((Minute / 10) + 48));
            builder.Append((char)((Minute % 10) + 48));
            builder.Append(":");
            builder.Append((char)((Second / 10) + 48));
            builder.Append((char)((Second % 10) + 48));

            return builder.ToString();
        }

        [Benchmark]
        public string Math_And_Prebuilt_Days()
        {
            var dayNumber = (int)Day;

            return "(" +
                (dayNumber + (Week * 7)) +
                ") " +
                PrebuiltDaysString[dayNumber] +
                "  " +
                (char)((Hour / 10) + 48) +
                (char)((Hour % 10) + 48) +
                ":" +
                (char)((Minute / 10) + 48) +
                (char)((Minute % 10) + 48) +
                ":" +
                (char)((Second / 10) + 48) +
                (char)((Second % 10) + 48);
        }

        [Benchmark]
        public string StringBuilder_Math_And_Prebuilt_Days()
        {
            var dayNumber = (int)Day;

            builder.Clear();
            builder.Append("(");
            builder.Append(dayNumber + (Week * 7));
            builder.Append(PrebuiltDaysString[dayNumber]);
            builder.Append((char)((Hour / 10) + 48));
            builder.Append((char)((Hour % 10) + 48));
            builder.Append(":");
            builder.Append((char)((Minute / 10) + 48));
            builder.Append((char)((Minute % 10) + 48));
            builder.Append(":");
            builder.Append((char)((Second / 10) + 48));
            builder.Append((char)((Second % 10) + 48));

            return builder.ToString();
        }
    }
}