Blackbaud have encrypted and restricted the use of Birthdate through Raiser's Edge. In doing so, they also re-formatted the Birth Date field. As a results, it is not currently possible to import Birthdate via Importacular, when you create a new template (existing templates, where Birthdate was previously mapped should be unaffected).
A fix has been been developed and should be available in version 6.2. (estimated release is Early May 2024)