A dated paper uses its own date. An undated one must not fall back to "now":
that value changes on every regeneration, so the entry looks freshly updated
each time the feed is built and every subscriber is re-notified forever. It
falls back instead to when the bytes were archived, which is recorded in the
record and therefore stable. fallback is the last resort for a record that
carries neither.
The instant an entry claims it was last updated.
A dated paper uses its own date. An undated one must not fall back to "now": that value changes on every regeneration, so the entry looks freshly updated each time the feed is built and every subscriber is re-notified forever. It falls back instead to when the bytes were archived, which is recorded in the record and therefore stable.
fallbackis the last resort for a record that carries neither.