Pluralize string (naive).cs

public static string Pluralize(this string str)
{
    if(str.EndsWith("s"))
        return str + "es";

    return str + "s";
}

QuantifyString.cs

public static string Quantify(this string str, double count)
{
    if(count == 1.0)
        return str;

    return str.Pluralize();
}

TimeSpanFormatter.cs

public static string Format(this TimeSpan span)
{
    double number;
    string unit;
    if (span.Days > 0)
    {
        number = span.TotalDays;
        unit = "day";
    }
    else if (span.Hours > 0)
    {
        number = span.TotalHours;
        unit = "hour";
    }
    else if (span.Minutes > 0)
    {
        number = span.TotalMinutes;
        unit = "minute";
    }
    else if (span.Seconds > 0)
    {
        number = span.TotalSeconds;
        unit = "second";
    }
    else
    {
        number = span.TotalMilliseconds;
        unit = "millisecond";
    }

    return string.Format("{0:0.#} {1}", number, unit.Quantify(number));
}