We use the salesShipOrder method to post shipping information to Magento2 from our fulfillment software. We're able to successfully create the shipment (along with carrier information, tracking numbers, customer notification), however there is not a way to indicate a shipment date (createdAt) for the package. When the post is made, it uses the current date/time as the shipment date, not the actual date/time that the package was shipped.
We began using the salesShipOrder method over the salesShipmentRepository method to combat the Magento2 bug where order status is not properly updated. However this new method does not include the essential element for shipment date.