It has been my understanding that we count 'up to' the fixed date (in accusative) until the Idus/ landing on each of those 3 fixed days in the ablative/but on reaching the Idus, we then count up to the subsequent Kalendae--i.e. March 16 = ante diem XVII kalendas apriles
It has been my understanding that we count 'up to' the fixed date (in accusative) until the Idus/ landing on each of those 3 fixed days in the ablative/but on reaching the Idus, we then count up to the subsequent Kalendae--i.e. March 16 = ante diem XVII kalendas apriles