RelativeTime.cs

public static class RelativeTime
{
    /// <summary>
    /// Returns a humanized string indicating how long ago something
    /// happened, eg "3 days ago". For future dates, returns when this
    /// DateTime will occur from DateTime.UtcNow.
    /// </summary>
    public static string ToRelativeTime(this DateTime dt)
    {
        var utcNow = DateTime.UtcNow;
        return dt <= utcNow ? ToRelativeTimePast(dt, utcNow) : ToRelativeTimeFuture(dt, utcNow);
    }

    private static string ToRelativeTimePast(DateTime dt, DateTime utcNow)
    {
        var ts = utcNow - dt;
        var delta = ts.TotalSeconds;

        if (delta < 60)
        {
            return ts.Seconds == 1 ? "1 sec ago" : ts.Seconds + " secs ago";
        }

        if (delta < 3600) // 60 mins * 60 sec
        {
            return ts.Minutes == 1 ? "1 min ago" : ts.Minutes + " mins ago";
        }

        if (delta < 86400) // 24 hrs * 60 mins * 60 sec
        {
            return ts.Hours == 1 ? "1 hour ago" : ts.Hours + " hours ago";
        }

        var days = ts.Days;
        if (days == 1)
        {
            return "yesterday";
        }

        if (days <= 2)
        {
            return days + " days ago";
        }

        if (days <= 330)
        {
            return dt.ToString("MMM %d 'at' %H:mmm");
        }

        return dt.ToString(@"MMM %d \'yy 'at' %H:mmm");
    }

    private static string ToRelativeTimeFuture(DateTime dt, DateTime utcNow)
    {
        var ts = dt - utcNow;
        var delta = ts.TotalSeconds;

        if (delta < 60)
        {
            return ts.Seconds == 1 ? "in 1 second" : "in " + ts.Seconds + " seconds";
        }

        if (delta < 3600) // 60 mins * 60 sec
        {
            return ts.Minutes == 1 ? "in 1 minute" : "in " + ts.Minutes + " minutes";
        }

        if (delta < 86400) // 24 hrs * 60 mins * 60 sec
        {
            return ts.Hours == 1 ? "in 1 hour" : "in " + ts.Hours + " hours";
        }

        // use our own rounding so we can round the correct direction for future
        var days = (int) Math.Round(ts.TotalDays, 0);
        if (days == 1)
        {
            return "tomorrow";
        }

        if (days <= 10)
        {
            return "in " + days + " day" + (days > 1 ? "s" : "");
        }

        if (days <= 330)
        {
            return "on " + dt.ToString("MMM %d 'at' %H:mmm");
        }

        return "on " + dt.ToString(@"MMM %d \'yy 'at' %H:mmm");
    }
}