Date instructions for dates without parser

[atomrab]

Since the ISO standard includes a zero year, and therefore 2 BC is -1, how do we make it clear to users who are entering BC dates in, for example, other languages (e.g. a.C., which doesn't parse), that they need to add positive 1 to all their BC dates?

On the same topic, our BP parsing has ignored the zero year (so 10000 BP parses as -8050). This is fine for real BP dates, which are usually so far in the past that the year doesn't matter. But because Fasti and now ARIADNE are using BP (2000) for all their dates, including more recent ones where a year in either direction will matter, should we insist on the zero year for this particular format? This is somewhat urgent issue, because we're about to start putting in ARIADNE dates and need to make sure we get this right.