8=VIII, 15=XV, 2003=MMIII. However, the Roman calendar differed to ours (some Emperors added in extra days to certain months for a start) so I doubt that they would have written the date as VIII/XV/MMIII. Depends on how accurate you want to be.
Found this site: http://www.guernsey.net/~sgibbs/roman.html
which will convert the date into a number of Roman formats. "Dies Veneris xv Augustus MMIII" is the date you mention in the Gregorian calendar in Latin. If you use the Julian calendar it comes out as August 2nd, or something that makes little sense to the non-historians among us in "Roman" style. Your best bet is probably the going to be the format above, or as in my previous post "VIII/XV/MMIII" or "XV/VIII/MMIII" if you prefer the day/month format.
Except the Romans wouldn't have written the date like that, they referred to dates as so many before the Kalends (1st of the month), the Nones (7th or 9th, depending on which month) or the Ides (13th or 15th) of the month. So 15th August would either be Ides Augusti MMIII (if the Ides fell on the 15th - something like Id. Aug. for short) or 15 ante Kalends Septembris MMIII (15 before the 1st September - or something like XV aK Sept).
But DIES VENERIS A.D. IV NON. AVG. MMDCCLVI A.U.C. isn't going to mean much to any non-historians, and "literally" translates as 2nd August (not taking variations in the calendar into account).