Convert 1d1h1m1s format to TimeSpan, and viceversa.

Durations.cs

void Main()
{
    Durations.FromString("10d").Dump();
    Durations.FromString("10h").Dump();
    Durations.FromString("10m").Dump();
    Durations.FromString("10s").Dump();
    Durations.FromString("100d10h10m10s").Dump();
    Durations.FromString("100d10h10s").Dump();
    Durations.FromString("0").Dump();

    Durations.ToString(TimeSpan.FromDays(10)).Dump();
    Durations.ToString(TimeSpan.FromHours(10)).Dump();
    Durations.ToString(TimeSpan.FromMinutes(10)).Dump();
    Durations.ToString(TimeSpan.FromSeconds(10)).Dump();
    Durations.ToString(new TimeSpan(100, 10, 10, 10)).Dump();
    Durations.ToString(new TimeSpan(100, 10, 0, 10)).Dump();
    Durations.ToString(TimeSpan.Zero).Dump();
}

public class Durations
{
    private const char DayChar = 'd';
    private const char HourChar = 'h';
    private const char MinuteChar = 'm';
    private const char SecondChar = 's';

    /// <summary>
    /// Convert 1d1h1m1s format to TimeSpan
    /// </summary>
    /// <param name="text"></param>
    /// <returns></returns>
    public static TimeSpan FromString(string text)
    {
        var dResult = ExtractNumber(text, DayChar);
        var hResult = ExtractNumber(dResult.leastText, HourChar);
        var mResult = ExtractNumber(hResult.leastText, MinuteChar);
        var sResult = ExtractNumber(mResult.leastText, SecondChar);
        return TimeSpan.FromSeconds(dResult.number * 24 * 60 * 60 + hResult.number * 60 * 60 + mResult.number * 60 + sResult.number);
    }

    /// <summary>
    /// Convert TimeSpan to 1d1h1m1s format
    /// </summary>
    /// <param name="input"></param>
    /// <returns></returns>
    public static string ToString(TimeSpan input)
    {
        var text = "";
        if (input.Days != 0)
            text += $"{input.Days}{DayChar}";
        if (input.Hours != 0)
            text += $"{input.Hours}{HourChar}";
        if (input.Minutes != 0)
            text += $"{input.Minutes}{MinuteChar}";
        if (input.Seconds != 0)
            text += $"{input.Seconds}{SecondChar}";

        if (string.IsNullOrEmpty(text))
            text = "0";

        return text;
    }

    private static (string leastText, int number) ExtractNumber(string text, char charactor)
    {
        if (string.IsNullOrEmpty(text))
            return ("", 0);

        var num = 0;
        var index = text.IndexOf(charactor);
        if (index != -1)
        {
            if (!int.TryParse(text.Substring(0, index), out num))
                throw new FormatException($"Invalid Format detected at {text}.");
            text = text.Substring(index, text.Length - index).TrimStart(charactor);
        }
        return (text, num);
    }
}