setInterval(function horarioAtual() {
let hora = new Date().toLocaleTimeString();
document.getElementById("rel").innerHTML = hora;
}, 1000)
<!DOCTYPE html>
<html>
<head>
<title>Parcel Sandbox</title>
<meta charset="UTF-8" />
</head>
<body>
<p>Relógio Digital - horário atual.</p>
<h2 id="rel"></h2>
<script src="src/index.js"></script>
</body>
</html>