You would need to echo the values to get them into the JS
<script>
var nowTime = <?php echo time(); ?>;
var startTime = <?php echo strtotime(get_field('start_date_time', false, false)); ?>;
var endTime = <?php echo strtotime(get_field('end_date_time', false, false)); ?>;
if (startTime <= nowTime && nowTime <= endTime) {
//do something
}
</script>
