(basic) Closure in Constructor Functon - what am i doing wrong?

Question

A private method is called to some private parameters. However, they log as “undefined”! The problem area is indicated with a in the code below. Why doesn’t Closure work here? What am I not getting?

(background: I am 3 weeks into learning JS and am making a simple timer as learning exercise)

Thanks in advance, Lenn

function Timer() {
    let _startTime , _stopTime, _durationHistory = 0;
    let _startTrigger = false;
    let _stopTrigger = false;

    timerSwitch = function(log, trigger) {
        log = Date.now() * 0.001;
        trigger = true; 
    };

    this.start = function(){
        timerSwitch(_startTime, _startTrigger); //why doesn't closure apply here? 
        console.log("timer started", _startTime); //the console logs _startTime as Undefined. I'm expecting it to inhert the new Date.now() *0.001
    };

    this.stop = function(){
        timerSwitch(_stopTime,_stopTrigger);
        _durationHistory = _durationHistory + (_stopTime - _startTime);
        console.log(`timer stopped: ${_stopTime}s | _durationHistory: ${_durationHistory}`);
    };

    this.reset = function(){
        _startTime = 0;
        _stopTime = 0;
        _durationHistory = 0;
        console.log('timer reset: start', _startTime, ' | stop ', _stopTime, '| history ', _durationHistory);
    };

    this.duration = function(){
        console.log(_durationHistory);
    };
};

tmr = new Timer();
tmr.start();```