Here’s a post that explains how to do date entry in the inspector without all the clicking. The Outliner should work similarly.
There is no way to inherit metadata, unfortunately, or specify a base date.
If you don’t get an answer here for this question, then create a test project, from the Blank template to keep it as simple as possible. Create a custom metadata Date field, add one document and update the value of the custom Date field. Then start poking through files in the test project’s .scriv folder until you find where the data for Custom date is stored. 
Best,
Jim