From a site on radiocarbon dating:

(“cal BC” means calibrated years before Christ as opposed to “BC” which means radiocarbon years before Christ.)

http://www.flmnh.ufl.edu/natsci/vertpaleo/aucilla12_1/radio9...



--------------------------------------------------
Note added at 19 mins (2009-07-07 20:18:39 GMT)
--------------------------------------------------

Note that some sources use BC cal., others cal BC
You might want to check with the client to see if they have a preference.

Two new radio carbon samples read 360 - 170 BC cal. and 180 to 50 BC calibrated.

http://www.harappa.com/baluch/print.html

To avoid confusion in reporting radiocarbon dates, the Journal of African Archaeology opted for standardized abbreviations as follows:
Uncalibrated radiocarbon years before present bp
Calibrated radiocarbon years BC, cal BC, AD or cal AD

http://www.african-archaeology.de/work/pdf/authors_guideline...

--------------------------------------------------
Note added at 21 mins (2009-07-07 20:20:45 GMT)
--------------------------------------------------

Final example:
Se intenta la convención, para ayudar un poco en el lío, de poner aC (o bC en inglés) para las fechas convencionales sin corregir, y AC (o BC, o incluso CAL AC o CAL BC) para las calibradas o corregidas, pero no todos los autores lo siguen.

http://www.celtiberia.net/articulo.asp?id=3347&pagina=2