Honestly, a leading zero in human-readable text is a non-solution. Year numbers are variable length decimal strings, not a four or five digit fixed length fiel

Honestly, a leading zero in human-readable text is a non-solution. Year numbers are variable length decimal strings, not a four or five digit fixed length field. Any historian concerned with 6 AD-999 AD will likely not include a leading zero in any year figures they use. I don't see why the people of the present or the future should do any different. Machine readable implementations, sure. Have as many digits as you wish. But for human written material, it just looks weird.