How do you extend the selection by 1 second on both sides of the selection?

No need for sorry. You already made a fantastic first draft in a zippy.

Maybe we need to use Pro Tools to do the time calculation. Meaning that we might need to skip the JavaScript and use keyboard keys to enter the selection fields. I thought it would be more safer to dive into JavaScript instead of the key simulations.

What I found, and also reported to the SoundFlow company, is that I think Pro Tools changed the way of working of the calculation method in the selection fields. The default "select between macros" action isn't working properly anymore in PT 2020.5. When entering a timecode in the selection field ("/") and use "+ 2 0 0 / " it doesn't automatically calculates anymore. You need to "enter" first and then go to the next field with "/". Unfortunately after the "enter" command the cursor exits the selection fields.

FWIW, I did make a function for adding and subtracting timecode.. I'm sure there is a slicker way to code it, but here it is if it helps anyone:

function timeCodeAddSubtract(time, offset) { //offset is whole number in seconds

    var hours = time.slice(0, 2);
    var minutes = time.slice(3, 5);
    var seconds = time.slice(6, 8);
    var frames = time.slice(09, 11);

    seconds = (+seconds + +offset);
    if (seconds < 0) {
        minutes = (+minutes - 1);
        seconds = (60 + seconds);
    } else if (seconds >= 60) {
        minutes = (+minutes + 1);
        seconds = (seconds - 60);
    }

    if (minutes < 0) {
        hours = (hours - 1);
        minutes = (60 + minutes);
    } else if (minutes >= 60) {
        hours = (hours + 1);
        minutes = (minutes - 60);

    }

    seconds = String(seconds);
    minutes = String(minutes);
    hours = String(hours);

    function AddZeros(value) {

        if (value.length == 1) value = String(`0${value}`);
        return value;
    }

    seconds = AddZeros(seconds);
    minutes = AddZeros(minutes);
    hours = AddZeros(hours);

    var output = (`${hours}:${minutes}:${seconds}:${frames}`);
    return output;
}