BCE years trigger ISO8601Error

[jonasengelmann]

I work with historical data and also BCE dates, however these currently result in an error, e.g.: Unrecognised ISO 8601 date format: '-0663-01-01'

Yet according to ISO 8601 BCE years should be denoted with a preceding - sign, e.g. -0002-04-12 is even listed as an example.