The reason BC is bad is because, when you hear BC, you think, oh this has been calibrated by knowledge of the passage of time according to records by humans. So

The reason BC is bad is because, when you hear BC, you think, oh this has been calibrated by knowledge of the passage of time according to records by humans. So perhaps we know it happened in year 20 of the reign of King B, and we have enough other information to believe that means about 125 BC. But the date of this event was determined by radioactive calibration. It's not measuring the same thing. It measures something that approximates human years. This might seem like a trivial distinction to lay people, but it's relevant to the field, and the 4.2 k BP year event is a technical term.