Hello Steven!
Thank you for your re-post!
Unfortunately the Store Manager does not allow to change the timestamp as it is automatically assigned by PrestaShop.
However the Store Manager has a built in Raw Table Editor, that allows to make changes directly to the database. You can find "ps_product" table which contains "date_add" and "date_upd" fields:
http://screencast.com/t/aqCoH6Fed
Important Note: we strongly recommend you to backup your database before making any changes in Raw Table Editor.
Let us know if you have any other questions.