cron job in wordpress

function wpse27856_set_content_type(){
    return "text/html";
}
add_filter( 'wp_mail_content_type','wpse27856_set_content_type' );

function myprefix_custom_cron_schedule( $schedules ) 
{
    $schedules['everysecond'] = array(
        'interval' => 1, // everysecond
        'display'  => __( 'Every second' ),
    );
    return $schedules;
}
add_filter( 'cron_schedules', 'myprefix_custom_cron_schedule' );

//Schedule an action if it's not already scheduled
if ( ! wp_next_scheduled( 'myprefix_cron_hook' ) ) {
    wp_schedule_event( time(), 'daily', 'myprefix_cron_hook' );
}

///Hook into that action that'll fire every six hours
 add_action( 'myprefix_cron_hook', 'myprefix_cron_function' );

//create your function, that runs on cron
function myprefix_cron_function() 
{
    email_notifications();
}

function email_notifications() 
{
    $subscriptions = get_posts( array(
        'post_type' => 'subscriptions',
        'posts_per_page' => -1,
        'meta_query' => array(
            array(
                'key' => 'ief_subscription_status',
                'value' => 'completed',
            )
        ),
    ) );
    
    if( $subscriptions && ! empty( $subscriptions ) && ! is_null( $subscriptions ) ) {
        foreach( $subscriptions as $subscription ) {
            $currrent_date = date( 'Y-m-d' );
            
            $start_subscription = get_field( 'subscription_date', $subscription->ID );
            $start_subscription_date = date( 'Y-m-d', strtotime( $start_subscription ) );
            
            $end_subscription = get_field( 'end_subscription_date', $subscription->ID );
            $end_subscription_date = date( 'Y-m-d', strtotime( $end_subscription ) );
            
            $remaining_days = date_diff( date_create( $currrent_date ),date_create( $end_subscription_date ) );
            
            $subscriber = get_userdata( get_field( 'subscriber_user_id', $subscription->ID ) );
            
            $course = get_post( get_field( 'course_id', $subscription->ID ) );

            /* 2 weeks before the end of subscription */
            if( $remaining_days->d == 15 ) {
                wp_mail(
                    $subscriber->user_email,
                    'Subscrption for '. $course->post_title,
                    'Hello '. $subscriber->display_name .'<br><br>Please be informed that your subscription for '. $course->post_title .' will expire on '. $end_subscription .' and you have '. $remaining_days->d .' days remaining.<br><br>Sincerely,<br>IEF'
                );
            }
            /* 1 week before the end of subscription */
            else if( $remaining_days->d == 8 ) {
                wp_mail(
                    $subscriber->user_email,
                    'Subscrption for '. $course->post_title,
                    'Hello '. $subscriber->display_name .'<br><br>Please be informed that your subscription for '. $course->post_title .' will expire on '. $end_subscription .' and you have '. $remaining_days->d .' days remaining.<br><br>Sincerely,<br>IEF'
                );
            } 
            /* 3 days before the end of subscription */
            else if( $remaining_days->d == 4 ) {
                wp_mail(
                    $subscriber->user_email,
                    'Subscrption for '. $course->post_title,
                    'Hello '. $subscriber->display_name .'<br><br>Please be informed that your subscription for '. $course->post_title .' will expire on '. $end_subscription .' and you have '. $remaining_days->d .' days remaining.<br><br>Sincerely,<br>IEF'
                );
            }
            /* 1 day before the end of subscription */
            else if( $remaining_days->d == 2 ) {
                wp_mail(
                    $subscriber->user_email,
                    'Subscrption for '. $course->post_title,
                    'Hello '. $subscriber->display_name .'<br><br>Please be informed that your subscription for '. $course->post_title .' will expire on '. $end_subscription .' and you have '. $remaining_days->d .' days remaining.<br><br>Sincerely,<br>IEF'
                );
            }
        }
    }
}