Hello (again) Angelo.
Apparently you can’t add a date element to the configuration form.
If you try it like this:
You get this exception:
<date translate="label comment">
This means that the date is missing the format. Unfortunately you cannot set the format by adding the <format> tag in the field configuration. That field is ignored.
Output format is not specified. Please, specify "format" key in constructor, or set it using setFormat().
Here is how I did it.
I made a copy of the file lib/Varien/Data/Form/Element/Date.php and placed it in app/code/local/Varien/Data/Form/Element/Date.php.
(I don’t know any other method to override a lib class).
Now my new class will be used instead of the one in lib.
Edit this new file and add this method:
This will ensure the format is never empty.
This will make the date format work only in theory.
There is still an issue with the calendar image that doesn’t appear.
I didn’t want to waste time on this so I’ve changed the trigger to the field itself.
In the same file look for the calendar setup section and change the trigger:
Now the calendar will appear when you click on the field.