Displaying member join date on page

You can use the WP_User_Query class to query on user meta, however, your specific example of querying for a list of users with a join date matching a specific month and from any year requires a query more complex than what you can do out of the box with the WP_User_Query class.

First, it’s important that the date format be correct and strictly enforced if you’re going to be querying on this data. I’d suggest using a jQuery datepicker widget to enable input of dates into the join date field. YYYY-MM-DD is the standard MySQL date format.

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EXAMPLE 1

Here’s an example of a simple query that will load all users who joined this month:

$first = new DateTime();
$first->modify( 'first day of this month' );
$first = $first->format( 'Y-m-d' );

$args = array(
    'meta_key' => 'join_date',
    'meta_value' => $first,
    'meta_compare' => '>=',
);

$user_query = new WP_User_Query( $args );

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EXAMPLE 2

Here’s a more complex meta query that will load users between specific dates:

$first = new DateTime();
$first->modify( 'first day of this month' );
$first = $first->format( 'Y-m-d' );
$last = new DateTime();
$last->modify( 'last day of this month' );
$last = $last->format('Y-m-d');

$args = array(
    'meta_query' => array(
        array(
            'key' => 'join_date',
            'value' => array( $first, $last ),
            'compare' => 'BETWEEN',
            'type' => 'DATE'
        )

    )
);
$user_query = new WP_User_Query( $args );

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EXAMPLE 3

For custom queries, you can modify the SQL directly. The first step is to set up a simple query:

$args = array(
    'meta_key' => 'join_date',
    'meta_value' => 0,
);

$user_query = new WP_User_Query( $args );
$users = $user_query->get_results();

foreach( $users as $user ):
    echo $user->user_nicename;
    echo ' joined ';
    echo get_user_meta( $user->ID, 'join_date', true );
endforeach;

Next, we add an action to pre_user_query to check for our simple query, and modify the SQL to just match the month and ignore year:

function wpse52318_user_query( $query ){
    if( $query->query_vars['meta_key'] == 'join_date' && $query->query_vars['meta_value'] === 0 ):
        global $wpdb;
        $thismonth = date('m');
        $query->query_where = "WHERE 1=1 AND ( $wpdb->usermeta.meta_key = 'join_date' AND ( MONTH( $wpdb->usermeta.meta_value ) = $thismonth ) )";
    endif;
    return $query;
}
add_action( 'pre_user_query', 'wpse52318_user_query' );