Adam Anderly

Yes, this works with the latest version of WooCommerce 2.3.7 (including 2.3.5).

Yes, you can simply use the hook ‘ss_wc_mailchimp_subscribe_merge_vars’ to modify the merge fields sent to MailChimp. See this post for an example: https://wordpress.org/support/topic/add-another-field?replies=9#post-6560225

It looks like you are using this plugin: https://wordpress.org/plugins/woocommerce-mailchimp-casl/

Mine is https://wordpress.org/plugins/woocommerce-mailchimp/

Not sure why this would be happening. Can you delete the plugin and install a fresh copy of the latest plugin version?

Please use this updated code instead of that above. The above code had the filter function parameters in the wrong order.

// First we add the new birthday field to the checkout fields
function ss_wc_add_birthday_field( $checkout_fields ) {

	$opt_in_checkbox_display_location = 'billing';

	$field_id = 'birthday';
	$field_name = __( 'Birthday', 'ss_wc_mailchimp' );

	$checkout_fields[$opt_in_checkbox_display_location][$field_id] = array(
		'type'    => 'text',
		'label'   => esc_attr( $field_name ),
		'placeholder' => 'mm/dd'
	);

	return $checkout_fields;
}
add_filter( 'woocommerce_checkout_fields', 'ss_wc_add_birthday_field' );

// Make birthday a date picker mm/dd field
function ss_wc_add_checkout_field_datepicker() {
    if ( is_checkout() ) {
        $js = 'jQuery( "#birthday" ).css("width","125px").datepicker({ changeMonth: true, changeYear: true, yearRange:"-100:+0", dateFormat: "mm/dd" });';
        // Check if WC 2.1+
        if ( defined( 'WC_VERSION' ) && WC_VERSION ) {
            wc_enqueue_js( $js );
        } else {
            global $woocommerce;
            $woocommerce->add_inline_js( $js );
        }
    }
}
add_filter( 'wp_footer' , 'ss_wc_add_checkout_field_datepicker' );

// Save the birthday field with the order
function ss_wc_save_birthday_field( $order_id ) {
	$birthday = isset( $_POST['birthday'] ) ? $_POST['birthday'] : null;
	update_post_meta( $order_id, 'birthday', $birthday );
}
add_action( 'woocommerce_checkout_update_order_meta', 'ss_wc_save_birthday_field' );

// tie into the WooCommerce MailChimp 'ss_wc_mailchimp_subscribe_merge_vars' action filter to add the MailChimp merge var for Birthday
function ss_wc_send_birthday_field_to_mailchimp( $merge_vars, $order_id ) {
    // retrieve the birthday from the order meta/custom fields
    $birthday = get_post_meta( $order_id, 'birthday', true );

    // Add 'BIRTHDAY' merge variable to MailChimp call (change 'BIRTHDAY' to whatever your merge variable is called in MailChimp)
	$merge_vars['BIRTHDAY'] = $birthday;
    return $merge_vars;
}
add_filter( 'ss_wc_mailchimp_subscribe_merge_vars' , 'ss_wc_send_birthday_field_to_mailchimp', 10 , 2 );

Just pushed version 1.3.6 to fix this problem.

Marking this as resolved as this is a known enhancement request.

These attributes are not exported to MailChimp, but this is a highly requested feature. I’d like to include this capability in a future release.