schedule-a-popup.php

<?php
function schedule_my_popup( $is_loadable, $popup_id ) {
	  if( $popup_id == 123 ) {
		
		    $start = strtotime( 'May 4, 2016 11:59AM' );
		    $end = strtotime( 'May 5, 2016 11:59AM' );
		    $now = strtotime( 'now' );
		    
		    if( $now >= $start && $now <= $end ) {
			      return true;
		    }
		    
		    else {
			      return false;
		    }
		    
	  }
	
	  return $is_loadable;
}
add_filter( 'popmake_popup_is_loadable', 'schedule_my_popup', 1000 );