Need Interpretation of timeInstant and timeDuration

I'm writing the timeInstant and timeDuration validators for Xerces-Java and need
some clarification.

Is a negative timeInstant meant to represent time B.C.?  I'm presuming "yes".

What sign would you assign to the following timeDuration results:

"CCYY-MM-DDThh:mm:ss.sss/PnYnMnDTnHnMnnS"  presume positive.
"CCYY-MM-DDThh:mm:ss.sss/-PnYnMnDTnHnMnnS"  presume negative.
"-CCYY-MM-DDThh:mm:ss.sss/PnYnMnDTnHnMnnS"  presume positive.
"-CCYY-MM-DDThh:mm:ss.sss/-PnYnMnDTnHnMnnS"  presume negative.
"PnYnMnDTnHnMnnS/CCYY-MM-DDThh:mm:ss.sss"  presume negative.
"PnYnMnDTnHnMnnS/-CCYY-MM-DDThh:mm:ss.sss"  presume negative.
"-PnYnMnDTnHnMnnS/CCYY-MM-DDThh:mm:ss.sss"  presume positive.
"-PnYnMnDTnHnMnnS/-CCYY-MM-DDThh:mm:ss.sss"  presume positive.

The assumptions are based on the idea that if the resultant time instant occurs
after the base instant then the duration is positive and if a resultant time
instant occurs before the base instant then the duration is negative.  Please
confirm or correct.

I'm assuming that you meant to include right-truncated forms timeInstant and
didn't mean for it to ALWAYS be expressed down to seconds to 3 decimal places.

Thanks,
George Joseph