> The "agreement between the sender and the receiver" makes it seem we
> don't need to document that we don't support signs on the years.
Agreement needs each side to state its terms, though, and PostgreSQL
states its terms in the documentation. It already does for the other
by-agreement range: ISO 8601 leaves years before 1583 to agreement,
and B.6 settles them - proleptic Gregorian for all dates - if you
think to look in an appendix on calendar history. For a leading sign
there is only B.1's tokenizing rule, "either a numeric time zone or
a special field", which says nothing about years. So a client finds
out by trying: -0001-01-02, the wiki's own 2 BC, is refused as
invalid input syntax, and +10000-01-02 as a time zone displacement.
