this is only one possibility
// current unix time stamp
$now = time();
// get field with no formatting
// this will return date time as stored in the db
// to that there isn't any problem with the date format
// when calling strtotime()
$start = strtotime(get_field('start_date_time', false, false));
// same for end
$end = strtotime(get_field('end_date_time', false, false));
if ($start <= $now && $now <= $end) {
// in the middle, do something.
}