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

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.