To say today's date, January 17, you would have to know
- what today is with reference to the main dating points in the Roman month,
- what case to put the various words in,
- what are their appropriate numbers and genders, and
- which word modifies which.
The Romans used inclusive counting. One technique for deriving the Roman-style inclusive counting number is to take the number of days in the current month (31) add 2 (31+2=33) and subtract today's date's number (17), so you get 31+2-17=16. That means that on the Roman calendar, it is 16 days until the Kalends of February.
The Roman calendar abbreviation for today's date is a.d. xvi. Kal. Feb. Here a.d. doesn't stand for anno domini, but ante diem 'before the day': ante diem xvi kalendas februarias, where xvi is the ordinal for 16 sextusdecimus. You need to decline sextusdecimum (a Latin 2nd Declension adjective) to agree with the masculine accusative singular of diem.