Stop WP All Import from changing the modified dates on import and, optionally, set the modified date.

wp-all-import-modified-dates.php

<?php
/**
 * These functions hook into the WP All Import plugin to control the post_modified date values.
 *
 * Such that you can avoid updating the modified date when importing, as well as explicitly set it.
 *
 * @since 2025-06-19
 **/

// Exit if accessed directly.
if ( ! defined( 'ABSPATH' ) ) {
	exit;
}

// Block modified date changes on imports.
add_action( 'pmxi_before_post_import', 'keep_modified_date_on_import' );

/**
 * Adds a filter before the import to block updating the modified date.
 *
 * We hook into the core filter within the pmxi_before_post_import filter
 * so we are only adding this logic to imports.
 *
 * @param int $import_id
 * @return void
 */
function keep_modified_date_on_import( $import_id ) {
	add_filter( 
		'wp_insert_post_data', 
	   function ( $new, $old ) {
			$new['post_modified']     = $old['post_modified'];
			$new['post_modified_gmt'] = $old['post_modified_gmt'];
			return $new;
		}, 
		10,
		2
	);
}

// Set modified dates from import data, if they exist.
add_action( 'pmxi_saved_post', 'set_post_modified_dates', 10, 3 );

/**
 * This will update the modified date, if it's provided in the import.
 *
 * To set a modified date, you must have a `post_modified` column with the new date
 * in the correct format: `YYYY-MM-DD HH:MM:SS`
 *
 * @see https://www.wpallimport.com/documentation/action-reference/#pmxi_saved_post
 *
 * @param int              $post_id   The post id of the imported post.
 * @param SimpleXMLElement $xml_node  The libxml resource of the current XML element.
 * @param bool             $is_update True if this is an update, false if it's a new post.
 * @return void
 */
function set_post_modified_dates( $post_id, $xml_node, $is_update ) {

	// Only run on updates.
	if ( ! $is_update ) {
		return;
	}

	// Convert SimpleXml object to array for easier use.
	$record = json_decode( json_encode( ( array ) $xml_node ), 1 );

	// Bail if we don't have what we need.
	if ( ! $post_id || ! $record || ! isset( $record['post_modified'] ) ) {
		return;
	}
	
	// Compare the modified date to the published date,
	// if the modified date is before the published date, we'll use the published date instead.
	$published_date = get_post_field( 'post_date', $post_id, 'raw' );
	if ( strtotime( $record['post_modified'] ) < strtotime( $published_date ) ) {
		$record['post_modified'] = $published_date;
	}

	// Assuming your import file has <post_modified> and <post_modified_gmt> columns:
	$modified_date = mysql2date( 'Y-m-d H:i:s', sanitize_text_field( $record['post_modified'] ), false );

	// Only run if we have a valid date.
	if ( $modified_date ) {
		wp_update_post(
			array(
				'ID'                 => $post_id,
				'post_modified'      => $modified_date,
				'post_modified_gmt'  => get_gmt_from_date( $modified_date ),
			),
			false, // don't return an error
			false // don't fire hooks on update
		);
	}
}