Everyone knows that the soothsayer in William Shakespeare’s Julius Caesar was talking about March 15 when he warned the Roman emperor to “beware the Ides of March.” We also all know Caesar’s response: “Nah, I gotta head into the office that day.” But if March 15 is the Ides of March, what does that make March 16?

At the time of Caesar’s assassination, Romans were using the Julian calendar (introduced by Julius Caesar himself; some thanks he got). This was a modified version of the original Roman calendar, and it is very similar to the one we use today (which is called the Gregorian calendar). A major difference, however, was how Romans talked about the days.

Each month had three important dates: the Kalends (1st of the month), the Ides (the middle of the month), and the Nones (9th day before the Ides, so roughly the quarter-mark of the month). Instead of counting up (i.e., March 10, March 11, March 12), Romans kept track by counting backwards and inclusively from the Kalends, Ides, or Nones. March 10 was the 6th day before the Ides of March, March 11 was the 5th day before the Ides of March, and so on.

Because it came after the Ides, March 16 would’ve been referred to in the context of April: "The 17th day before the Kalends of April." The abbreviated form of this was a.d. XVII Kal. Apr., with “a.d.” standing for ante diem, meaning roughly “the day before.”

So, had Julius Caesar been murdered on March 16, the soothsayer’s ominous warning would have been, “Beware the 17th day before the Kalends of April.” Doesn’t have quite the same ring to it.