WordPress - force a specific /YYYY/MM folder for media uploads.

wp-force-subdir-on-upload_dir.php

<?php

const FORCE_UPLOAD_DIR_SUBDIR_TO = '/1990/08'; // MUST be "/YYYY/MM" format.

/**
 * Force uploads to land in a specific /YYYY/MM folder.
 *
 * @param array $uploads information about the upload directory
 * @see https://github.com/WordPress/WordPress/blob/2b92bcab85cecd819596b79b0b52f44aa4dfaffa/wp-includes/functions.php#L2223-L2239
 * @return array
 */
function force_custom_upload_dir_subdir( $uploads ) {
	if ( ! get_option( 'uploads_use_yearmonth_folders' ) ) {
		return $uploads;
	}

	// in case some weird subdir path is prepended, extract the date.
	preg_match( "(\/(19|20)\d{2}\/(0|1)\d{1})", $uploads['subdir'], $current_subdir_date );

	if ( $current_subdir_date[0] ) {
		foreach ( [ 'subdir', 'path', 'url' ] as $k ) {
			$uploads[$k] = str_replace( $current_subdir_date[0], FORCE_UPLOAD_DIR_SUBDIR_TO, $uploads[$k] );
		}
	}

	return $uploads;
}

add_filter( 'upload_dir', 'force_custom_upload_dir_subdir' );