Dates in Latin (Roman Numbers)


Civis Illustris
Today is the Nonae (Nonis). After the Nonae it's always a viii Id. (that's why the Nonae are called Nonae).
Now, it's November, a 30-day long month, so after the Idus it'll be the even number among 17, 18 and 19. So, it'll be xviii K. Dec.

When the Idus is on the 13th and the month has 31, it'll be the xix K. (menses sequentes). If the Idus is on the 15th, the next day will be the xvii K. (m.s.). It's that easy!