How to caluculate the difference between two times?

public string getFormattedTimeFromSecond(double second)
    {

        TimeSpan t = TimeSpan.FromSeconds(second);

        string formatedTime = string.Format("{0:D2}H:{1:D2}M:{2:D2}S",
                                t.Hours,
                                t.Minutes,
                                t.Seconds);

        return formatedTime;
}

DateTime currentDate = DateTime.Now;
DateTime postedDate = DateTime.Now.AddHours(-2);
TimeSpan ts = postedDate.Subtract (currentDate);