Not surprisingly, chakka35, we have a disagreement... not uncommon is such discussions, no? Fact is there is no AD1 or BC 1 until the completion of that year. For all of either year, we may wish to call it 1BC or AD1 but it is neither. It could only be a fraction of the 12 month period. And, as far as attribution of a "nought" (love the British expressions) to a man's birthday, you yourself explain that just such a "nought" is the "infintesimal moment on the borderline between 1BC and AD 1"... I, of course, maintain that there is no such "infinitesimal moment' because AD1 or BC1 are separated by 24 months. Each year on either side is Year Zero ,until 12 months has elapsed. Wikipedia agrees, saying "...This means that between, for example, 500 BC, January 1 and AD 500, January 1 there are 999 years: 500 for the time taking place BC, and 499 for the part of AD. common usage Anno Domini 1 is preceded by the year 1 BC, without an intervening year zero..." However, if a person on this side of the dviding line talks about something happening 6 months BC they cannot be referencing 1BC simply because that hasn't occured yet.
Now, if we used Ordinal numbers instead of Cardinal numbers, some (but not all) of the problem would be aleviated...