Problem:
DatePicker (Example: Author-Date of birth, Date of death) is disabled by default during entity creation (when date-input is empty).

Cause:

 src/client/entity-editor/common/new-date-field.js

empty year value evaluates to '0' for isCommonEraDate value, making 'disabled=true' , for both negative and empty years.

Fix:
You need to modify the isCommonEraDate value checking, so that it evaluates to 'true' for both '1' & '0' ( currently it's only checking for value '1' ).