Removing leading zeros from custom permalink structure

How about using custom rewrite tags/structure?

So we’ll be using these two rewrite/structure tags:

1. %monthnum2% does the same thing as the %monthnum% tag, but without a leading zero; e.g. 3 and not 03 for March.

2. %day2% does the same thing as the %day% tag, but without a leading zero; e.g. 7 and not 07.

The steps:

1. In the theme functions file (functions.php), add:

   add_action( 'init', function(){
       add_rewrite_tag( '%monthnum2%', '([0-9]{1,2})', 'monthnum=' );
       add_rewrite_tag( '%day2%',      '([0-9]{1,2})', 'day=' );
   } );
   

   That will generate the %monthnum2% and %day2% (rewrite) tags and be used when WordPress (re-)generates the rewrite rules.

   And then add this:

   add_filter( 'post_link', function( $permalink, $post ){
       if ( preg_match( '/%(?:monthnum2|day2)%/', $permalink ) ) {
           $time = strtotime( $post->post_date );
           $date = explode( ' ', date( 'n j', $time ) );
   
           $permalink = str_replace( [
               '%monthnum2%',
               '%day2%',
           ], [
               $date[0],
               $date[1],
           ], $permalink );
   
           $permalink = user_trailingslashit( $permalink, 'single' );
       }
       return $permalink;
   }, 10, 2 );
   

   That will replace the rewrite tags in the permalink.

2. Go to the permalink settings page and then in the “Custom Structure” box, enter this structure: /%year%/%monthnum2%/%day2%/%postname%/ or /%author%/%year%/%monthnum2%/%day2%/%postname%, whichever applies.

   The point is, use %monthnum2% to display the month number without a leading zero and %day2% to display the day number without a leading zero.

3. Save your changes — both the theme functions file and the permalink settings — and go to the “Posts” page (wp-admin/edit.php) and just check everything (mouse-over the post permalink and visit the post).