Are you sure that the field is returning nothing or could stringtotime()
not be evaluating the returned value correctly?
<?php while ( have_posts() ) : the_post(); ?>;
<?php // skip expired posts
$expirationtime = get_field("expiration");
// try outputting the values
echo $expirationtime,': ',strtotime($expirationtime);
if ($expirationtime) {
$secondsbetween = strtotime($expirationtime)-time();
if ($secondsbetween <= 0) continue;
}
?>;