Format relative time in CSharp

public static string TimeAgo(DateTime dt)
{
    TimeSpan span = DateTime.Now - dt;

    if (span.Days > 365)
    {
        int years = (span.Days / 365);
        if (span.Days % 365 != 0)
            years += 1;
        return String.Format("{0} {1} ago",
        years, years == 1 ? "year" : "years");
    }

    if (span.Days > 30)
    {
        int months = (span.Days / 30);
        if (span.Days % 31 != 0)
            months += 1;
        return String.Format("{0} {1} ago",
        months, months == 1 ? "month" : "months");
    }

    if (span.Days > 0)
        return String.Format("{0} {1} ago",
        span.Days, span.Days == 1 ? "day" : "days");
    if (span.Hours > 0)
        return String.Format("{0} {1} ago",
        span.Hours, span.Hours == 1 ? "hour" : "hours");
    if (span.Minutes > 0)
        return String.Format("{0} {1} ago",
        span.Minutes, span.Minutes == 1 ? "minute" : "minutes");
    if (span.Seconds > 5)
        return String.Format("{0} seconds ago", span.Seconds);
    if (span.Seconds <= 5)
        return "just now";

    return string.Empty;
}