How to get a list of all users registered before a given date?

by

Method #1: The pre_user_query hook:

There are not many filters available, but you can try the pre_user_query hook:

// Add filter:
add_action(    'pre_user_query', 'wpse_filter_by_reg_date' );

// Query:
$query = new WP_User_Query( $args );

// Remove filter:
remove_action( 'pre_user_query', 'wpse_filter_by_reg_date' );

where the filter callback is:

/**
 * Filter WP_User_Query by user_registered date
 *
 * @see   http://wordpress.stackexchange.com/a/160826/26350
 * @param WP_User_Query The current WP_User_Query instance
 */

function wpse_filter_by_reg_date( $q )
{
    global $wpdb;
    $q->query_where .=  $wpdb->prepare(
        " AND {$wpdb->users}.user_registered >= '%s' ",
        date( "Y-m-d" )
    );
}

You can then modify the filter callback to your needs.

Method #2: WP_User_Query with custom query variables:

You can also make this more dynamic, by using the custom string parameters _registered and _registered_compare. Then your query could be:

$args = array(
    'orderby'             => 'login',
    'order'               => 'ASC',
    '_registered'         => date( 'Y-m-d' ),
    '_registered_compare' => '>=',
);
$query = new WP_User_Query( $args );

where:

/**
 * Add support for the custom '_registered' and '_registered_compare'
 * string input parameters in WP_User_Query().
 *
 * @see   http://wordpress.stackexchange.com/a/160826/26350
 * @param WP_User_Query The current WP_User_Query instance
 */

function wpse_registered_filter( $q )
{
    if( isset( $q->query_vars['_registered'] )
        && isset( $q->query_vars['_registered_compare'] )
        && is_string( $q->query_vars['_registered_compare'] )
    )
    {   
        // Input:
        $registered = $q->query_vars['_registered'];
        $compare    = $q->query_vars['_registered_compare'];

        // Init:     
        $available_compares = array( '=', '<', '>', '<=', '>=', '!=' );

        // Default compare:
        if( ! in_array( $compare, $available_compares, TRUE ) )
            $compare="=";

        // Modify query:
        global $wpdb;                                               
        $q->query_where .=  $wpdb->prepare(
            " AND {$wpdb->users}.user_registered {$compare} '%s' ",
            $registered
        );
    }
}
add_action( 'pre_user_query', 'wpse_registered_filter' );

Method #3: Manual SQL:

Responding to the comment below, this is an example how you can write the SQL manually:

    global $wpdb;
    $sql = $wpdb->prepare(
        "SELECT ID FROM {$wpdb->users} WHERE {$wpdb->users}.user_registered >= '%s' ",
        current_time( 'mysql' )
    );
    $uids = $wpdb->get_col( $sql );