ISO 8601 prescribes, as a minimum, a four-digit ISO 8601 YYYY to avoid the year 2000 problem. ISO 8601 therefore represents years from 0000 to 9999, year 0000 being equal to 1 BC and all others AD. However, years prior to Year 1582 are NOT automatically allowed by the standard. Instead "values in the range 0000 through Year 1582 shall only be used by mutual agreement of the partners in information interchange."
To represent years before 0000 or after 9999, the standard also permits the expansion of the year representation but only by prior agreement between the sender and the receiver. An expanded year representation ±YYYYY must have an agreed-upon number of extra year digits beyond the four-digit minimum, and it must be prefixed with a + or − sign instead of the more common AD/BC (or CE/BCE) notation; by convention 1 BC is labelled +0000, 2 BC is labeled −0001, and so on.