Advanced Custom Fields and date picker, show posts only if the day is today no matter the year

It’s a little complicated, but we can do this by directly modifying the query with a filter on posts_where.

We’ll start with a query for posts with a meta value of today’s date, this is assuming a date format of yyyy-mm-dd:

$date_today = date('Y-m-d');
$args = array(
    'posts_per_page' => -1,
    'meta_query' => array(
        array(
            'key' => 'died_date',
            'value' => $date_today,
            'compare' => '=',
            'type' => 'DATE'
        )
    )
);

$results = new WP_Query( $args );

If we look at the WHERE clause of this query, we’ll see the relevant bit:

AND ( (wp_postmeta.meta_key = 'died_date'
AND CAST(wp_postmeta.meta_value AS DATE) = '2013-02-14') )

To get this to work, we want to modify the last line to use MySQL’s DAYOFYEAR in the comparison.

So here’s our filter:

function wpa86916_filter_where( $where ){
    global $wpdb;
    $date_today = date('Y-m-d');
    $parts = explode( 'AND CAST', $where );
    $new_where = " AND DAYOFYEAR( CAST( $wpdb->postmeta.meta_value AS DATE ) ) = DAYOFYEAR( '$date_today' ) ) )";
    $where = $parts[0] . $new_where;
    remove_filter( 'posts_where', __FUNCTION__ );
    return $where;
}

If we back up to our original query, we can now add this filter before the query, and we should get all posts with matching day of year, regardless of year:

add_filter( 'posts_where', 'wpa86916_filter_where' );
$results = new WP_Query( $args );

As for your question regarding extraction of the year from a date, we can use PHP’s date and strtotime to do any date extraction and conversion:

$date="2013-02-14";
echo date( 'Y', strtotime( $date ) );
// prints '2013'

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