How do I convert a TimeSpan to a formatted string?

I just built a few TimeSpan Extension methods. Thought I could share:

public static string ToReadableAgeString(this TimeSpan span)
{
    return string.Format("{0:0}", span.Days / 365.25);
}

public static string ToReadableString(this TimeSpan span)
{
    string formatted = string.Format("{0}{1}{2}{3}",
        span.Duration().Days > 0 ? string.Format("{0:0} day{1}, ", span.Days, span.Days == 1 ? string.Empty : "s") : string.Empty,
        span.Duration().Hours > 0 ? string.Format("{0:0} hour{1}, ", span.Hours, span.Hours == 1 ? string.Empty : "s") : string.Empty,
        span.Duration().Minutes > 0 ? string.Format("{0:0} minute{1}, ", span.Minutes, span.Minutes == 1 ? string.Empty : "s") : string.Empty,
        span.Duration().Seconds > 0 ? string.Format("{0:0} second{1}", span.Seconds, span.Seconds == 1 ? string.Empty : "s") : string.Empty);

    if (formatted.EndsWith(", ")) formatted = formatted.Substring(0, formatted.Length - 2);

    if (string.IsNullOrEmpty(formatted)) formatted = "0 seconds";

    return formatted;
}

By converting it to a datetime, you can get localized formats:

new DateTime(timeSpan.Ticks).ToString("HH:mm");