I'm using postgres.
In my data model I've a date_a (DATE) field and using it in DateRange dashboard filter.
Then I changed the DB by adding a new DATE field "date_new." In admin section I ran the sync and rescan actions. But still when I want now to map this date_new field to the DateRange filter, the drop down shows only the originally defined date_a field. The new field is not in the list.
Depending on how you have created the question, adding a new column to your database might not automatically make the new field appear, so you'll have to go to the question's Notebook editor and click the arrow next to the table to make sure you also include that field.
Otherwise try making a new question and see if that works.
Thanks @flamber, I investigated the surrounding and found my mistake. The mapping to the new date field is done in the "variables" dialog of the Question Editor. I thought at first it is done in the dashboard edit dialog directly.