Can I schedule a preorder to start or end automatically? (Essent Preorder Back in Stock app)

Yes, on the Lite plan or higher.

1. Open the campaign → Content tab → Status, scheduling → choose Scheduled.
2. Tick Set start date and pick the date and time. Leave it unticked to start now.
3. Tick Set end date to stop it automatically.
4. Save. Times follow your store's timezone.

When a campaign ends, the button goes back to Add to cart or Sold out depending on stock. An ended campaign doesn't restart by itself. Click Publish to run it again.

End a preorder by hand

Click Unpublish on the campaign rather than deleting it. When asked, choose to turn off "Continue selling when out of stock" so sold-out products don't keep selling. Existing orders aren't affected.

Full guide: Campaign status: Live, Scheduled and Draft.