Well, but I wouldn't type the date this way. Romans did it quite different way.
There were some special days, called Kalendae, Nonae, and Idus. The first day of each month was called Kalendae (calends) while the fifth day or, in months of 31 days, the seventh day, was called Nonae (nones). Finally the 13th or 15th day, respectively, was named Idus (ides), marking more or less the middle of the month. In between these kind of lables, the Romans counted the days backwards, including the "lable days". With this system, the second day of Ianuarius was designated "day IIII before the nones of Ianuarius" (ANTE DIEM IIII NONAS IANVARIAS).
But as it is a date from our period, I'd type it in modern system:
die XXIX mensis Septembris, anno post Christum natum MMIX.