Trial Accounts That Downgrade After a Set Number of Days

// Role during the trial, role afterwards, and trial length.
$wpide_trial_role  = 'contributor';
$wpide_after_role  = 'subscriber';
$wpide_trial_days  = 14;

add_action( 'user_register', function ( $user_id ) use ( $wpide_trial_role ) {
    if ( current_user_can( 'create_users' ) || ! get_role( $wpide_trial_role ) ) {
        return;
    }

    ( new WP_User( $user_id ) )->set_role( $wpide_trial_role );
    update_user_meta( $user_id, 'wpide_trial_started', time() );
}, 20 );

add_action( 'init', function () use ( $wpide_trial_role, $wpide_after_role, $wpide_trial_days ) {
    $user_id = get_current_user_id();
    $started = $user_id ? (int) get_user_meta( $user_id, 'wpide_trial_started', true ) : 0;

    if ( ! $started || time() - $started < $wpide_trial_days * DAY_IN_SECONDS ) {
        return;
    }

    $user = wp_get_current_user();

    if ( in_array( $wpide_trial_role, (array) $user->roles, true ) ) {
        $user->remove_role( $wpide_trial_role );
        $user->add_role( $wpide_after_role );
    }

    delete_user_meta( $user_id, 'wpide_trial_started' );
    update_user_meta( $user_id, 'wpide_trial_ended', time() );
} );

add_shortcode( 'wpide_trial_days_left', function () use ( $wpide_trial_days ) {
    $started = is_user_logged_in() ? (int) get_user_meta( get_current_user_id(), 'wpide_trial_started', true ) : 0;

    if ( ! $started ) {
        return '';
    }

    $left = max( 0, (int) ceil( ( $started + $wpide_trial_days * DAY_IN_SECONDS - time() ) / DAY_IN_SECONDS ) );

    return esc_html( sprintf( _n( '%d day left in your trial', '%d days left in your trial', $left ), $left ) );
} );