using System;

using static System.Console;

public class Program {

    public static void Main() {

        var d = new DateTime(2014, 1, 15);

        var d2 = new DateTime(2015, 7, 15);

        var d3 = new DateTime(2015, 8, 25, 8, 0, 0);

        var d4 = new DateTime(2015, 8, 24);

        var agora = new DateTime(2015, 8, 25, 0, 0, 0);

        WriteLine((d - agora).RelativeTime());

        WriteLine((d2 - agora).RelativeTime());

        WriteLine((d3 - agora).RelativeTime());

        WriteLine((d4 - agora).RelativeTime());

    }

}

​

public static class RelativeTimeExtensions {

    public static String RelativeTime(this TimeSpan ts) {

        const int second = 1;

        const int minute = 60 * second;

        const int hour = 60 * minute;

        const int day = 24 * hour;

        const int month = 30 * day;

        double delta = Math.Abs(ts.TotalSeconds);

        //melhor se escrever só "Agora há pouco"

        if (delta < 1 * minute) return "Há " + (ts.Seconds == 1 ? "um segundo" : ts.Seconds + " segundos");

        if (delta < 2 * minute) return "Há um minuto";

        if (delta < 45 * minute) return "Há " + ts.Minutes + " minutos";

        if (delta < 90 * minute) return "Há uma hora";

        if (delta < 24 * hour) return "Há " + ts.Hours + " horas";

        if (delta < 48 * hour) return "ontem";

        if (delta < 30 * day) return "Há " + ts.Days + " dias";

        if (delta < 12 * month) {

            var months = Convert.ToInt32(Math.Floor((double)ts.Days / 30));

            return "Há " + (months <= 1 ? "um mês" : months + " meses");

        } else {

            var years = Convert.ToInt32(Math.Floor((double)ts.Days / 365));

            return "Há " + (years <= 1 ? "um ano" : years + " anos");

        }

    }

}