How to save time Interval before is ends?

Question

Hello!

I have Form with a Timer1, and it set to 10Sec.

There is a KeyDown event - when the user press "Enter", I would like to save in "ans" the time duration that past in the 10S interval before it ends.

For Example:  If i starting the timer1 now, and after 3Sec, I'm pressing Enter, ans = 3. and if I didnt press any key, ans will be equal to 10.

I have this code:

        if (e.KeyCode == Keys.Enter)
        {
            ResponseTimeList.Add(timer1.Interval);
        }

*ResponseTimeList is:

    public List<double> ResponseTimeList = new List<double>();

How can i improve it?

Thanks.