This question is an exact duplicate of:
- DateTime iso format in api 1 answer
Applying the date_iso8601 Drupal function in a simple process_callbacks on the public_fields in the class file for each date field works to convert the output/display of the date format, but does not allow that format to be used for filtering by the date value.
'process_callbacks' => array( 'date_iso8601', ),
So we need to figure out how to make the query filter use the ISO 8601 date format and support things like partial matches using just YYYY-MM-DD and ensure different operators work as well (>, <, >=, <=).