Problem/Motivation
The date field currently hard-codes a dc:date in core/modules/datetime/lib/Drupal/datetime/Plugin/Field/FieldFormatter/DatetimeDefaultFormatter.php:126. We need to support the RDF mappings defined by core.
Proposed resolution
Change core/modules/datetime/lib/Drupal/datetime/Plugin/Field/FieldFormatter/DatetimeDefaultFormatter.php to use the proper mappings of core. A good example is the taxonomy field formatter at #1778122-113: Change notice: Enable modules to inject attributes into field formatters, so that RDF attributes get output .
Remaining tasks
write patch
User interface changes
none
API changes
none