<div data-time='<?=time();?>' id ='clock'></div>
<script type="text/javascript">
    setInterval(() => {
        const clock = document.getElementById('clock');
        let time = Number(clock.dataset.time);
        clock.textContent = new Date((time+1) * 1000).toISOString().slice(10, 19).replace('T', ' ');
        clock.dataset.time = (time+1);
    }, 1000);
</script>