JSFiddle - React, Tailwind, and code Playground

HTML

<p id="time"></p>
<button id="startstop">Start/stop</button>

JavaScript

// assuming you declared ti and t out here, cuz I hope they're not global
var t = 0;
var ti;    
var time = null;
var running = false;

function startpause() {
    
    clearTimeout(ti);
    if(!running) {
        ti = setTimeout(tim,1000); 
        running = true;
    } else {
        running = false;
    }
}
function tim() { 
    time.innerHTML = t;
    t = t + 1; 
    ti = setTimeout(tim,1000); 
}

$(document).ready(function() {
    $('#startstop').click(startpause);
    time = $('#time')[0];
});