C# today, yesterday, last week, last month

public static string ToTimeSinceString(this DateTime value)
{
    const int SECOND = 1;
    const int MINUTE = 60 * SECOND;
    const int HOUR = 60 * MINUTE;
    const int DAY = 24 * HOUR;
    const int MONTH = 30 * DAY;

    TimeSpan ts = new TimeSpan(DateTime.Now.Ticks - value.Ticks);
    double seconds = ts.TotalSeconds;

    // Less than one minute
    if (seconds < 1 * MINUTE)
        return ts.Seconds == 1 ? "one second ago" : ts.Seconds + " seconds ago";

    if (seconds < 60 * MINUTE)
        return ts.Minutes + " minutes ago";

    if (seconds < 120 * MINUTE)
        return "an hour ago";

    if (seconds < 24 * HOUR)
        return ts.Hours + " hours ago";

    if (seconds < 48 * HOUR)
        return "yesterday";

    if (seconds < 30 * DAY)
        return ts.Days + " days ago";

    if (seconds < 12 * MONTH) {
        int months = Convert.ToInt32(Math.Floor((double)ts.Days / 30));
        return months <= 1 ? "one month ago" : months + " months ago";
    }

    int years = Convert.ToInt32(Math.Floor((double)ts.Days / 365));
    return years <= 1 ? "one year ago" : years + " years ago";
}