Filter post listing by meta value which is a date

Add a query variable to store the month

So first of all it’ll be necessary to create a custom query variable – this will store the month we’re after (in 2012/6) format. I’ll call it custom_month, but its best practise to prefix it to avoid clashes with other plug-ins:

add_filter('query_vars', 'wpse57344_register_query_vars' );
function wpse57344_register_query_vars( $qvars ){
    //Add these query variables
    $qvars[] = 'custom_month';
    return $qvars;
}

Add the drop-down

The query variable will now be recognised and any given value stored by WordPress in the main query (global $wp_query).

Next we create the drop-down to go at the top of the post admin table (this can be changed to be any post type).

add_action( 'restrict_manage_posts', 'wpse57344_restrict_posts_by_metavalue' );
function wpse57344_restrict_posts_by_metavalue() {
    global $typenow;
    $months = wpse57344_get_months();
    if ($typenow == 'post') {
        $selected = get_query_var('custom_month');
        $output = "<select style="width:150px" name="custom_month" class="postform">\n";
        $output .= '<option '.selected($selected,0,false).' value="">'.__('Show All','wpse57344_plugin').'</option>';
        if ( ! empty( $months ) ) {
            foreach ($months as $month):
                $value =esc_attr($month->year."https://wordpress.stackexchange.com/".$month->month);
                $month_dt = new DateTime($month->year.'-'.$month->month.'-01');
                $output .= "<option value="{$value}" ".selected($selected,$value,false).'>'.$month_dt->format('F Y').'</option>';
            endforeach; 
        }
        $output .= "</select>\n";       
    echo $output;
    }
}

The above gets an array of months a month object consists of:

year => //the year of the month e.g. 2012
month => //the number of the month, e.g. 6 for July
posts => //number of posts with the date meta value in this month

Obviously wpse57344_get_months() doesn’t exist natively – we’ll construct it later.

Assuming with an array of months, we create the drop-down with each option having the value of the form yyyy/mm. I’ve given the form the same name as the query variable we added.

Altering the query

When a month in the drop-down is selected, the month in yyyy/mm is posted as the value for custom_month. Because we’ve registered this variable name with WordPress we can access it through $query->get('custom_month').

So we check if its empty or not – if its not, then we restrict to posts where their meta value data is in that month. To do that we use a meta query and the BETWEEN operator.

add_action( 'pre_get_posts', 'wpse57351_pre_get_posts' );
function wpse57351_pre_get_posts( $query ) {

    //Only alter query if custom variable is set.
    $month_str = $query->get('custom_month');
    if( !empty($month_str) ){

            //For debugging, uncomment following line
            //var_dump($query);

        //Be careful not override any existing meta queries.
        $meta_query = $query->get('meta_query');
        if( empty($meta_query) )
            $meta_query = array();

        //Convert 2012/05 into a datetime object get the first and last days of that month in yyyy/mm/dd format
        $month = new DateTime($month_str.'/01');

        //Get posts with date between the first and last of given month
        $meta_query[] = array(
            'key' => 'customdate',
            'value' => array($month->format('Y/m/d'),$month->format('Y/m/t')),
            'compare' => 'BETWEEN',
        );
        $query->set('meta_query',$meta_query);

            //For debugging, uncomment following line
            //var_dump($query);
    }
}

Get the months

It remains then to define th function wpse57344_get_months(). We need to query the postmeta table and pick out distinct months from the dates in your posts’ meta. In the following I assume that your date is stored with key ‘customdate’ and has format (as indicated in comments) yyyy/mm/dd.

function wpse57344_get_months(){
    global $wpdb;
        $months = wp_cache_get( 'wpse57344_months' );
        if ( false === $months ) {
            $query = "SELECT YEAR(meta_value) AS `year`, MONTH(meta_value) AS `month`, count(post_id) as posts 
                FROM $wpdb->postmeta WHERE meta_key ='customdate'           
                GROUP BY YEAR(meta_value), MONTH(meta_value) ORDER BY meta_value DESC";
            $months = $wpdb->get_results($query);
            wp_cache_set( 'wpse57344_months', $months );
        }
        return $months;
}

This returns an array of month objects in the desired format.