Time standard BC AD?

[wxdqz]

I'm about to write a DTD for Numismatics (coin-collecting) and wonder ifthere is a standard for dates (years) BC and AD. like age of an emperor:10BC-AD15and if unsecure date, how do i do that? my own solution is kind of like for10?BC-AD10:<date><post type="unsecure" time="BC">10</post><pre type="secure" time="AD">10</pre></date>i don't want them in just a string <date>10?BC-AD10</date> since I need tobe able to make date-manipulations.very grateful for any tips/apotygma