RFC 3339 date-time string with any timezone offset (RFC 8620 §1.4).
Format: YYYY-MM-DDTHH:MM:SS±HH:MM or Z suffix — any valid RFC 3339 offset,
letters uppercase, fractional seconds omitted if zero.
Example: "2014-10-30T14:12:00+08:00".
Distinct from UTCDate, which requires the time-offset to be Z.
Use Date for fields derived from RFC 5322 email headers (e.g. sentAt),
which commonly carry non-UTC offsets.
Date is #[serde(transparent)] over String. Any string that
deserializes into a String deserializes into a Date, including
values that violate RFC 8620 §1.4 / RFC 3339. The newtype carries
the type-level intent but does NOT enforce it at the wire
boundary. Treat any field of type Option<Date> arriving from a
peer as “RFC 8620 timestamp by convention, not by contract”.