Summary:
You are free to edit the future publish date of the app from your developer portal account after your app has been submitted. From the Apps & Services page, click to edit the current version of the app and go to the Availability & Pricing tab, click on the Edit button on the lower right and change the publish date. Once submitted, your changes will take affect shortly. However, please note that the field will not be editable if the date originally entered has already passed.
