app/views/admin/comparisons/reports/_display_custom_period.html.erb
<script type="text/javascript">
$(document).ready(function(){
$('.report_reporting_period').on('change', function() {
const selected = $(this).find(":selected").val();
const display = (selected == 'custom') ? 'block' : 'none';
$('.custom_period').css('display', display);
}).trigger('change');
})
</script>