How to create a default `creationDate` for a document in sanity.io?

Question

I have checked the documentation of how to do this, but I have not found an answer.

Basically, when a document is created/published, I want to set a createdDate for the document (as a read-only/hidden field).

Answer 1

I found the answer here.

At its core, a document is a JSON-object that has a unique _id, timestamps (_createdAt, _updatedAt) and revision-marker _rev.

So the created date is generated automatically by sanity, including many other interesting properties.

Answer 2

As mentioned in svale's comment, it is possible to set both _createdAt and _updatedAt but only upon creation. I've been using Sanity's official JS client and it allows me to do this:

await client.create({
    _type: "someType",
    _createdAt: "2019-12-31T12:34:56Z",
    _updatedAt: "2020-01-01T12:34:56Z",
});

Additionally, it's possible to set a custom _id if needed.

When updating a document, those attributes are indeed read-only and cannot be manually changed.