They have not added this feature yet (very important IMHO), though they hinted at sometime this year…
In the meantime, the best route is to use a hook with something like addPostWebhook. Keep in mind, for the moment it only supports post-hooks (hopefully pre-hooks one day). This should be up on Cloud DGraph by the end of the week with 21.03:
Basically, you would just add a createdAt value after the rest of the field is created, and you could add updatePostWebhook with updatedAt value for after a field is updated. If someone passed in a date, you could reject the call, or simply replace the value with the correct date and not worry about it.
J