What are the Kalends, Nones, and Ides?

The Romans counted days backward from three fixed points each month: the Kalends (1st), the Nones (5th, or 7th in March/May/July/October), and the Ides (13th, or 15th in those same four months).

How is 'ante diem' counting done?

The Romans counted inclusively. 'ante diem III Kalendas' means the 3rd day before the Kalends, counting the Kalends itself as day one — so actually 2 days before the Kalends in modern counting.

What is a Julian Day Number?

The Julian Day Number (JDN) is a continuous count of days since noon on January 1, 4713 BC used by astronomers and historians to avoid calendar ambiguity.

Does this handle the Julian calendar as well as Gregorian?

Yes. You can switch between Gregorian and Proleptic Julian calendar systems to handle historical dates before the 1582 Gregorian reform.

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Roman / Julian Calendar Converter

Convert Gregorian dates to Roman date notation and Julian Day Numbers.

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About this tool

Translate any modern date into authentic Roman calendar notation — for example, 'ante diem III Kalendas Martias' (3 days before the Kalends of March). Understand the three reference points: Kalends (1st), Nones (5th or 7th), and Ides (13th or 15th) of each month. Shows the Roman month names (Ianuarius through December), the day of the week in classical Latin (dies Solis, Lunae, Martis, Mercurii, Iovis, Veneris, Saturni), and the Julian Day Number used in astronomy. Convert in both directions — enter a Roman date string or a Julian Day Number to get the Gregorian date.

How to use

1 Enter a Gregorian date using the date picker to see its Roman notation.
2 Read the explanation of which reference point (Kalends, Nones, or Ides) is used and why.
3 Check the Latin day-of-week name and the Julian Day Number shown below.
4 To convert back, type a Roman date string (e.g. 'a.d. III Kal. Mar.') into the reverse field.
5 Toggle the calendar system switch to compare Gregorian and Julian dates for historical periods.

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