function horarioAtual() {
let hora = new Date().toLocaleTimeString();
document.getElementById("rel").innerHTML = hora;
setTimeout(horarioAtual, 1000)
}
<!DOCTYPE html>
<html>
<head>
<title>Parcel Sandbox</title>
<meta charset="UTF-8" />
</head>
<body>
<p>Clique no botão abaixo para ver a hora atual.</p>
<button onclick="horarioAtual()">Clique aqui</button>
<h2 id="rel"></h2>
<script src="src/index.js"></script>
</body>
</html>