> The answer: Text.

That was my immediate thought too and led to me wondering: How do you represent BCE dates in ISO 8601?

Apparently ISO 8601 always supports YYYY from 0000 (1 BCE) to 9999 (9999 CE). ISO 8601 can also extend beyond those limits if agreed upon by sender and receiver: e.g. -0001 (2 BCE), -0002 (3 BCE), etc.