<div class="post-nav">
    <div class="nav-previous">
        <?php if ( is_a( get_previous_post() , 'WP_Post' ) ) : ?>
            <?php previous_post_link( '%link', '%title' ); ?>
            <span><?php echo get_the_date( 'Y.m.d' ); ?></span>
        <?php endif; ?>
    </div>
    <div class="nav-next">
        <?php if ( is_a( get_next_post() , 'WP_Post' ) ) : ?>
            <?php next_post_link( '%link', '%title' ); ?>
            <span><?php echo ( get_field( 'event_date' ) != '' ? get_field( 'event_date' ) : get_the_date( 'Y.m.d' ) ); ?></span>
        <?php endif; ?>
    </div>
</div>