Displaying custom posts only if custom meta box’s date is not expired

I have a problem with displaying custom posts – I want each post to show only if a date set in a custom field has not passed.

I can’t figure out the where I should put the if statement in the code. My idea is:

if($daysleft >= 0)
 then execute the while "custom post";

The following code is displaying the all the custom posts – both expired and not expired:

<?php               
while(have_posts() ) : the_post();
$date = get_post_meta($post->ID, '_single_date', true); 
$daysleft = round( ($date-time()) / 24 / 60 / 60);  ?>

<div class="coupon-date">
<?php
if($date == '')_e('valid', 'Couponize');
else if($daysleft <= 0) _e('expired', 'Couponize'); 
else echo sprintf( _n('%d day left.', '%d days left.', $daysleft, 'Couponize'), $daysleft ); 
?>

In your loop while(have_posts() ) : the_post(); you are unconditionally going through all posts, but since the loop stops after the_post() nothing actually happens. So that’s where the condition has to go:

while(have_posts() ) {
    the_post();
    if ($daysleft >= 0) {
         ... other actions
         }
    }

Your code looks a bit incomplete to me, so I’m not sure if my exact code will work, but this definitely is where you want the condition.

Please note that if the query has returned, for instance, 15 posts you may have less displayed if some of them are expired. If you want a fixed amount displayed, you must not change the loop which displays the result of the query, but the query itself.