Add row to custom database Table and delete all rows older than 1 day

You have got a few things mixed up.

First, your query checks for time_created, which does not exist in your table.

Second, you try to insert the date as a integer using %d. You may have to treat it as a string.

Finally, you have to set $wpdb to global in your function

function postviews_add_ip () {
    global $post, $wpdb; // Database set to global
        $id = $post->ID;
        $ip = $_SERVER['REMOTE_ADDR'];
        $time = date("Y-m-d H:i:s");

    $wpdb->insert( 
        'wp_postviews_ips', 
        array( 
            'postid' => $id,
            'ip' => $ip,
            'time' => $time
        ), // added a colon
        array(
            '%d',
            '%s', 
            '%s' // treat it as a string
        ) 
    );

    $sql = "DELETE FROM wp_postviews_ips WHERE time < (NOW() - INTERVAL 1 DAY)"; //changed rowname

    $wpdb->query($sql);
}

Placement of the code and structure

I would strongly recommend to divide the functions of insertion and deletion. You could use wp-cron for deletion, so you do not have to call this function all the time. The function itself should work this way – but please be sure to optimze it 🙂

Placement of the code and structure

Related Posts: