Javascript/jQuery Countdown Timer with Update Capability

I’ve seen several examples for Javascript/jQuery countdown timers. I have implemented a 15 minute countdown timer based off the example at w3schools. However, I would like to add an update feature that allows me to add/subtract time to and from the clock. The timer starts at 15 minutes.

In my project the user will answers questions. After they answer the question and select submit, a time penalty or bonus is given. The penalty subtracts 3 seconds from the clock. The bonus adds 2 seconds to it. Below is an example of my code:

var mission_timer = 15;

function start_timer(){
        
        countDownDate  = new Date().getTime() + mission_timer * 60 * 1000;
        timer_interval = setInterval(function() {

            var now                   = new Date().getTime();
            var distance              = countDownDate - now;
            var minutes               = Math.floor((distance % (1000 * 60 * 60)) / (1000 * 60));
            var minutes_number_length = Number(String(minutes).length);
            var seconds               = Math.floor((distance % (1000 * 60)) / 1000);
            var seconds_number_length = Number(String(seconds).length);
            
            current_time = document.getElementById("game_timer").innerHTML = "TIME: " + minutes + ":" + seconds;
            update_timer = null;
            time_update  = 0;
            
            if (current_time === "TIME: 00:00") {
                $("#game_timer").css("color", "red");
                clearInterval(timer_interval);
                console.log("Time Expired");
            }else
            if (current_time === "TIME: 01:00") {
                $("#game_timer").css("color", "yellow");
                console.log("One Minute Left");
            }
        
        }, 1000);
    }