I've done some searching and testing with the Shopify Fulfillment and FulfillmentOrder API and it doesn't seem possible to set a fulfillment date for an old order. We have a lot of customers that are importing orders that were created in the past. Many of these orders have fulfillment dates in the past, this can be an issue at times because some of the reports may be wrong or invoices they need to send to their customers may have the wrong fulfillment date.
The fulfillment has a created_at and updated_at field but both of these fields are read-only. Is there a way we can have merchants set the fulfillment date for orders in the past? I know this isn't part of a typical fulfillment process but we see this with a lot of merchants who are migrating from other platforms or merchants who are importing orders that may have been fulfilled from their brick and mortar store or another platform.