Kalendas

To create the intercalary day, the existing ante diem sextum Kalendas Martias (February 24) was doubled, producing ante diem bis sextum Kalendas Martias.

The days of these calendars were counted down (inclusively) to the next named day, so February 24 was ante diem sextum Kalendas Martias ("the sixth day before the calends of March") often abbreviated a. d. VI Kal.

The new leap day was dated as ante diem bis sextum Kalendas Martias ('the sixth doubled before the Kalends of March'), usually abbreviated as a.d. bis VI Kal.