CodeWars/4kyu/human_readable_duration_format/solution.cs

using System;
using System.Collections.Generic;
using System.Linq;

public class HumanTimeFormat{
  private static Dictionary<string, int> conversions = new Dictionary<string, int>() {
      {"year", 31536000},
      {"day", 86400},
      {"hour", 3600},
      {"minute", 60},
      {"second", 1}
  };

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

      var results = new List<string>();
      foreach (var pair in conversions) {
          var units = seconds / pair.Value;
          seconds %= pair.Value;
          if (units > 0) {
              var part = $"{units} {pair.Key}";
              if (units > 1) {
                  part += "s";
              }
              results.Add(part);
          }
      }

      if (results.Count == 1) return results[0];
      else {
          var last = results.Last();
          results.Remove(last);

          var result = String.Join(", ", results);
          return result + " and " + last;
      }
  }
}