Marks a member field of a document as the document's creation timestamp.
The creation timestamp is used for document expiry (see
Document.TtlMillis) and as one
of the sort options for queries.
This field is not required. If not present or not set, the document will be assigned the current timestamp as its creation timestamp.
If present, the field must be of type
See the class description of
Document for other requirements (i.e. if
present it must be visible, or have a visible getter and setter, or be exposed through a