By default, ISO 8601-1:2019 does not permit negative years in date representations.
Expanded representation (i.e. five digits or more for the year) permits both positive and negative years (and the sign is mandatory for both positive and negative years).
ISO 8601-2:2019 has provisions for numeric extensions. Subclause 4.4.1.2 permits negative four-digit years (years -9999 to -0001).
Note that both of these options require mutual agreement.
