"
 Why not simply follow the advice given on date sorting at Help:Sorting?  Here is an example taken from the first few entries in the table as of today that contains 3 BC years and 3 AD years that seems to be numerically sortable in the chronological (not lexical) manner one would hope for: 69.126.127.193  

 Year  City State or province CountryNotes !9299 700 BC  Ticul Yucatán Mexico !9499 500 BC  Cholula  Puebla Mexico !9699 300 BC Teotihuacan México Mexico In the Valley of Mexico 0200 200 Mitla Oaxaca Mexico 0650 650 Cahokia Illinois United States 1000 1000 Acoma Pueblo and Taos Pueblo New Mexico  United StatesOldest continuously occupied communities in the United States. The Acoma Pueblo today is known as Sky City. 1003 1003 L'Anse aux Meadows Newfoundland & Labrador CanadaThe Norse explorer Leif Ericson established a settlement at 51°N on this site in 1003.  This is considered to the first European contact with North America."