C# DateTime + TimeSpan = an extra 12 hours?

I want to log time spent on a job, and I would like to accumulate that time over many entries for work on the same job at different times of the day.

I have it recording time, and adding the time difference together and it works....except my output looks like this:

Original Time: 00:00:30
Addition Time: 00:00:10
TotalTime Out: 12:00:40

In this line of code:

totTime.Text = dt4.ToString("hh:mm:ss");

If I don't add the "hh:mm:ss" it adds up correctly, but includes the date which I don't really want. If that helps to narrow it down.

Anyone know why it is adding 12 hours here? I can't see a reason for it to do so at all.

My code to get the time entries and calculate the differences looks like this:

public void startbutton_Click(object sender, EventArgs e)
        {

            startTime.Text = DateTime.Now.ToString("hh:mm:ss");

        }

public void stopbutton_Click(object sender, EventArgs e)
        {

            stopTime.Text = DateTime.Now.ToString("hh:mm:ss");

            DateTime dt1 = DateTime.ParseExact(startTime.Text, "hh:mm:ss", new DateTimeFormatInfo());
            DateTime dt2 = DateTime.ParseExact(stopTime.Text, "hh:mm:ss", new DateTimeFormatInfo());

            TimeSpan ts1 = dt2.Subtract(dt1);

            diffTime.Text = ts1.ToString();

            if (String.IsNullOrEmpty(totTime.Text))
            {
                totTime.Text = ts1.ToString();
            }
            else if (totTime.Text != "")
            {
                DateTime dt3 = DateTime.ParseExact(diffTime.Text, "hh:mm:ss", new DateTimeFormatInfo());

                DateTime dt4 = dt3 + ts1;
                totTime.Text = dt4.ToString("hh:mm:ss");
            }

Thanks.