Description:

Your task in order to complete this Kata is to write a function which formats a duration, given as a number of seconds, in a human-friendly way.

The function must accept a non-negative integer. If it is zero, it just returns "now". Otherwise, the duration is expressed as a combination of years, days, hours, minutes and seconds.

It is much easier to understand with an example:

* For seconds = 62, your function should return 
    "1 minute and 2 seconds"
* For seconds = 3662, your function should return
    "1 hour, 1 minute and 2 seconds"

For the purpose of this Kata, a year is 365 days and a day is 24 hours.

Note that spaces are important.

Detailed rules

The resulting expression is made of components like 4 seconds, 1 year, etc. In general, a positive integer and one of the valid units of time, separated by a space. The unit of time is used in plural if the integer is greater than 1.

The components are separated by a comma and a space (", "). Except the last component, which is separated by " and ", just like it would be written in English.

A more significant units of time will occur before than a least significant one. Therefore, 1 second and 1 year is not correct, but 1 year and 1 second is.

Different components have different unit of times. So there is not repeated units like in 5 seconds and 1 second.

A component will not appear at all if its value happens to be zero. Hence, 1 minute and 0 seconds is not valid, but it should be just 1 minute.

A unit of time must be used "as much as possible". It means that the function should not return 61 seconds, but 1 minute and 1 second instead. Formally, the duration specified by of a component must not be greater than any valid more significant unit of time.

My solution

using System;
using System.Text;

public class HumanTimeFormat
{
    public static string formatDuration(int seconds)
    {
        if (seconds == 0)
            return "now";

        var timeSpan = TimeSpan.FromSeconds(seconds);
        int year = timeSpan.Days / 365;
        timeSpan = timeSpan.Subtract(TimeSpan.FromDays(year * 365));
        int count = 0;

        if (year > 0) ++count;
        if (timeSpan.Days > 0) ++count;
        if (timeSpan.Hours > 0) ++count;
        if (timeSpan.Minutes > 0) ++count;
        if (timeSpan.Seconds > 0) ++count;

        var result = new StringBuilder();

        result.Append(FormatConclusion(year, "year", ref count));
        result.Append(FormatConclusion(timeSpan.Days, "day", ref count));
        result.Append(FormatConclusion(timeSpan.Hours, "hour", ref count));
        result.Append(FormatConclusion(timeSpan.Minutes, "minute", ref count));
        result.Append(FormatConclusion(timeSpan.Seconds, "second", ref count));

        return result.ToString();
    }

    public static string FormatConclusion(int value, string timeInterval, ref int count)
    {
        if (value == 0)
            return "";

        if (value != 1)
            timeInterval += "s";

        return count-- switch
        {
            > 2 => $"{value} {timeInterval}, ",
            2 => $"{value} {timeInterval} and ",
            _ => $"{value} {timeInterval}"
        };
    }
}