<script>
function imtc_calculation() {
alert('imtc_calculation');
}
</script>
<button onclick="imtc_calculation();">CLICK</button>
<ul id="test">
<li>1</li>
<li>2</li>
<li>3</li>
<li>4</li>
</ul>
.red {
color: red;
}
const hours = new Date().getHours();
if (hours >= 9 && hours <= 15) {
document.querySelector('#test li:nth-child(3n)').classList.add('red')
}