function timer() {
    let running = false;

    $('#start-timer').click(startTimer);
    $('#stop-timer').click(stopTimer);

    let hours = $('#hours').text();
    let minutes = $('#minutes').text();
    let seconds = $('#seconds').text();

    function startTimer() {

        if (!running) {
            running = true;
            timer = setInterval(step, 1000);

            function step() {

                if (seconds >= 59) {
                    seconds = 0;
                    minutes++;
                    $('#minutes').text(minutes < 10 ? ('0' + minutes.toString()) : minutes);
                }
                if (minutes >= 59) {
                    minutes = 0;
                    hours++;
                    $('#hours').text(hours < 10 ? ('0' + hours.toString()) : hours);
                }
                seconds++;
                $('#seconds').text(seconds < 10 ? ('0' + seconds.toString()) : seconds);
            }
        }
    }

    function stopTimer() {
        if (running) {
            running = false;
            clearInterval(timer);
        }
    }
}