Changing date query from month

I’m kinda confused on why you’re passing 0 as meta_value, but if your storing unix timestamps under anniversary_date, this approach should be sufficient:

// queries anniversary dates between today and the next 30 days
    $query = new WP_Query(array(
        'order' => 'ASC',
        'orderby' => 'meta_value',
        'meta_key' => 'anniversary_date',
        'meta_value' => array(strtotime('today'), strtotime('+30 days')),
        'meta_compare' => 'BETWEEN'
    ));