wp-cron creates backup problems

Hi,

We’ve recently let some of our users install WordPress, but this is causing us problems with backups. wp-cron appears to change the modification time of the wp-content/themes and wp-content/plugins directories but doesn’t change any of the contents. Unfortunately, this is identified as a filesystem modification by our backup software, and an incremental backup is generated, even when not needed.

Is there a way to avoid this, and still use the wp-cron, which I understand is important for updates and other things?

Thank you!
Scott