Function format_timestamp 

pub fn format_timestamp(at: Timestamp) -> String

One instant, RFC 3339, for a place that is not a struct field.

#[serde(with = "time::serde::rfc3339")] is the answer wherever there is a field to attach it to. This is the answer where there is not — inside a json! literal, where a bare Timestamp would take the derived component array with nothing in the source to hint at it. See Timestamp.

Falls back to the numeric Unix second if formatting fails, which needs an instant outside RFC 3339’s representable range. That is not a case worth failing an effect key over, and a number is at least unambiguous.