Adding time on button click to a count down timer

1. Im creating a countdown timer which starts at 3mins and 30secs.
2. When the timer reaches 0 the initial 3:30 timer will be repeated.
3. This happens until the user presses a button, which will add 1:45 to the timer and pause the timer until the user decides to resume the timer from the new value. Eg ( 3:30 + 1:45 = 5:15).

Now I have got the first 2 step to work with my current code, but I'm having a lot of issues with the 3rd part. Once the user clicks the add 1.45 button the count works, but only up until a certain point. After this point it will start to display a negative integer.

I'm sure there is an easier way to write this code. I have really overcomplicated this. Any suggestions would be appreciated.

//Define vars to hold time values
let startingMins = 3.5;
let time = startingMins * 60;

//define var to hold stopwatch status
let status = "stopped";
//define var to holds current timer
let storeTime = null; 
//define Number of sets
let setNum = 1;

//Stop watch function (logic to determin when to decrement each value)
function stopwatch () {
    minutes = Math.floor(time / 60);
    let seconds = time % 60;
    seconds = seconds < 10 ? '0' + seconds : seconds;
    storeTimer = minutes + ":" + seconds; //Store time in var
    storeTime = minutes + "." + seconds; //Store time in var

    //Display updated time values to user
    document.getElementById("display").innerHTML = storeTimer;
    time--;

    // When timer reachers 0 secs the inital 3:30 countdown will begin again.
    if (time <= 0) {
        startingMins = 3.5;
        time = startingMins * 60;
        minutes = Math.floor(time / 60);
        seconds = time % 60;
        seconds = seconds < 10 ? '0' + seconds : seconds;
        setNum++;
        //console.log(setNum);
    }
}


function StartStop() {
    if(status === "stopped") {
        //start watch
        interval = window.setInterval(stopwatch, 100);
        var startButton = document.getElementById("start");
        document.getElementById("start").innerHTML = "Pauce";
        //startButton.style.display = "none"
        status = "start";
        //console.log(time);
    }
    else { 
        window.clearInterval(interval);
        document.getElementById("start").innerHTML = "Start";
        status = "stopped";
        console.log(storeTime);
    }
}

function pauceAdd () {

    if(status === "stopped") {
        //start watch
        interval = window.setInterval(stopwatch, 1000);
        var zukButton = document.getElementById("pauceAdd");
        status = "start";
    }
    else { 
        window.clearInterval(interval);
        status = "stopped";
        console.log("store time " + storeTime);
        let time = +storeTime + +1.45; //store time is 3.30 adding 4.75
        console.log("store time2 " + time); // correct result 4.75 
        minutes = Math.floor(time);/// convert time from Mins (4.75) to seconds (475)
        let seconds = time % 60; // 5
        
        if (seconds < 60 ) { // if the Stored time is greater than 60 secs add 1 minute to the timer
            minutes++;
            seconds = seconds * 100; 
            console.log("secs updated = " + seconds ); // seconds updated (475)
            if (seconds <= 460) { 
                seconds = Math.floor(seconds - 460); 
                console.log("seconds 2 == " + seconds)
            }
            else if (seconds > -60) { // Stuck here
                seconds = seconds + 60;// Stuck here
            }// Stuck here
            else {
                seconds = Math.floor(seconds - 460);
                console.log("seconds 2 = " + seconds)
            }
        }
        if (seconds < 1) {
            seconds = seconds + 60;
            minutes = minutes - 1;
        }

        seconds = seconds < 10 ? + seconds : seconds;
        console.log("mins updated = " + minutes + "__________________________-");

        //Display updated time values to user
        document.getElementById("display").innerHTML = minutes + ":" + seconds;
    }
}

function reset () {
    //window.clearInterval(storeTime);
    window.clearInterval(interval);
    startingMins = 3.5;
    time = startingMins * 60;
    minutes = Math.floor(time / 60);
    seconds = time % 60;
    seconds = seconds < 10 ? '0' + seconds : seconds;
    status = "stopped";
    setNum = 1;
    var startButton = document.getElementById("start");
    startButton.style.display = "inline-block";
    document.getElementById("display").innerHTML = "3:30";
    document.getElementById("start").innerHTML = "Start";
}