Which calendar are these dates in?

Short answer: each event appears on the day the Romans themselves recorded. Each entry carries a tag telling you which calendar that day belongs to.

1. The Republican calendar (before 45 BC)

The old Roman year had 355 days. The pontiffs were supposed to keep it in step with the seasons by inserting an extra month (intercalation) every other year or so. They often did this badly or for political reasons, so by Caesar's time the calendar had drifted far from the seasons. Suetonius says the harvest festivals no longer fell in summer, nor the vintage festivals in autumn. Events tagged Pre-reform Roman calendar are dated in this calendar, and we don't convert them because the drift in any given year is usually unknown.

2. Caesar's reform (46–45 BC)

Julius Caesar set the year at 365 days with one extra day every fourth year, starting 1 January 45 BC. To realign it, 46 BC was stretched to 445 days. In Latin, Suetonius writes that he "fastos correxit… annumque ad cursum solis accommodavit" ("corrected the calendar… and fitted the year to the course of the sun"; Suetonius, Julius 40). Macrobius (Saturnalia 1.14.2–13) and Censorinus (De Die Natali 20.8–11) also describe the reform.

3. The Augustan leap-year fix (to AD 8)

After Caesar's death the priests added leap days every third year by mistake. Augustus suspended leap years for a time; regular every-fourth-year leap days resumed by AD 8. For 45 BC–AD 8 the exact match between a Roman day and a day in today's reckoning is uncertain by a day or two, so those entries are marked approximate. (Julian calendar, Wikipedia.)

4. Julian vs Gregorian (our calendar)

The Julian year is about 11 minutes too long, so it slowly drifts. In 1582 Pope Gregory XIII introduced the Gregorian calendar we use today. Today a Julian date is 13 days behind the Gregorian. In Roman times the gap was small: about 2 days the other way around the turn of the era, shrinking to 0 in the 3rd century AD and becoming +1 in the 4th. For Julian-dated events we show the equivalent day in the Gregorian calendar extended backwards (the "proleptic" Gregorian), worked out by Julian Day Number. Example: Tiberius was born on 16 November 42 BC (Julian), which is about 14 November on our calendar.

5. Why we use as-recorded dates

An "on this day" site is about anniversaries as the Romans counted them: the Ides of March is 15 March. Converting every date would cut events off from their sources, inscriptions and festivals, and for pre-reform dates it would mean guessing. So we keep the recorded date and label the calendar.

Tags you'll see

• Julian date · ≈ [day] on our calendar: from 45 BC onward, with the modern (proleptic Gregorian) equivalent.
• Pre-reform Roman calendar: Republican dates before 45 BC; seasonal date unknown.
• Roman calendar date: annual festivals with no single year.

See also: how Kalends, Nones and Ides work.