try something like this
<?php
$found = false;
if (have_rows('dates')) {
while(have_rows('dates')) {
the_row();
if ($value == get_sub_field('custom_date')) {
$found = true;
echo 'true';
}
} // end while have rows
} // end if have rows
if (!$found) {
echo 'false';
}
?>
