Insert a countdown in each node based on a pre-existing date field
I assume you have two fields in one content type - countdown_date and expiration_date. Both values user has to insert manually. And after submit action, you want to assign expiration date to both fields, yes?

Read about hook_node_submit(). In your MYMODULE_node_submit($node, $form, &amp;$form_state) you can check the type of content, then assign expiration_date to countdown_date. This method works with save and preview's action.

For more detailed answer I need more details about these fields.