Pre-get-post function to sort post by ACF date field.

Functions.php

<?php

function register_my_menus() {
  register_nav_menus(
    array(
      'header-menu' => __( 'Header Menu' ),
      'extra-menu' => __( 'Extra Menu' )
    )
  );
}
add_action( 'init', 'register_my_menus' );


function my_pre_get_posts( $query ) {
	
	// do not modify queries in the admin
	if(  $query->is_main_query() && !is_admin() ) {
		
		return $query;
		
	}
	

	// trier par date de deadline
	else
	{	
		$query->set('orderby', 'meta_value_num');	
		$query->set('meta_key', 'deadline');	 
		$query->set('order', 'ASC'); 
				
	}
	

	// return
	return $query;

}

add_action('pre_get_posts', 'my_pre_get_posts');

?>