display date and time on my header

Hi there,

Do you currently have the navigation set to float right?

yes

Try this snippet:

add_action( 'generate_inside_navigation','generate_date_navigation' );
function generate_date_navigation()
{
    ?>
    <span class="nav-date">
        <?php the_date(); ?>
        <?php the_time(); ?>
    </span>
    <?php
}

Adding PHP: https://docs.generatepress.com/article/adding-php/

Or put a widget like this into the header widget: https://en-ca.wordpress.org/plugins/date-and-time-widget/