Manage post modified date values in WP All Import

This snippet allows you to block changing the post’s “modified date” when running imports as well as explicitly set the `post_modified` date if you have a column for it in your csv. Be sure to use the sql date format YYYY-MM-DD HH:MM:SS

Source: https://gist.github.com/joshuadavidnelson/d260b2e07bf22d434534f29927748b35