Does anybody know a formula, trick, or gods forbid, a friggin' (free) website or program for generating a Gregorian calendar for a BCE year? Like, say I wanted to know what day of the week Caesar was assassinated; how'd I go about it and determine that?

This should account for the "misaligned" leap years, as 1 BCE, 5 BCE, 9 BCE, etc. would be leap years, I guess, rather than years divisible by four (and the skipping of leap days in centuries, e.g., 2000, would also be off by one). You can't just use 44 CE, in this example, as a template because of that, plus the shift in days is wrong -- March 15th in one year would be a day later the next, but of course a day earlier the previous year; they're not mirror images.


OP here, following up after posting another question, but don't have the Rep Points to comment.

@Gerard_Ashton Well, if it was called March 15 in ancient Rome, but we don't how many days exist between then and now, it might not be March 15 as we know it now, anyway, right?

@Mark_Johnson Very handy; thanks. I was going to do that over that weekend, but then dan04 came along with his answer. ;)

@Pieter_Geerkens No, I'm well aware that the calendar and dates are man-made as plainly evidenced by being concerned with the Gregorian calendar and the mention of leap days, just as I am with the time, as every year at the change of Daylight Savings, I'm explaining to someone, 'no, it's not really 2:00 instead of 3:00 any more than it's 'the hour of the tiger,' or Theta, or Red.' Anyway, I'm not trying to base any historical or academic argument on it (like, 'it was a Sunday, which is why Friday the 13th is unlucky!') It's just an interesting bit of trivia, like what day this space exploration thing happened, a book was published, Guy Fawkes tried to blow up Parliament, or Oreo cookies went on sale, even if, after millennia, it doesn't wash as entirely accurate.

@dan04 @Don Al As @DJClayworth suggests, this would be exactly the sort of thing I was looking for. Thanks!! Sorry that I can't upvote you, but #respect.

@Torsten Thanks for that, if I do get to programming, maybe for mobile application or desktop program.

@Johannes Ooh; I like that! It has the Mayan calendar (though maybe Aztec would be even cooler, but that can probably be easily converted). If I get very sticklery about it, I'll use that to make sure it's correct (such as it maybe can be), but again, it's mostly just a trivia thing.

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    This webpage seems to have a number of calendar converters, if you're willing to pay for one.
    – Steve Bird
    Jul 24, 2023 at 19:04
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    You can't find out what day of the week Caesar was assassinated because the records of how the Julian calendar was actually observed in Rome are incomplete. So we know he died on 15 March 44 BC as it was called in Rome back then, but we don't know how many calendar days it is from, for example, the death of Caesar to 24 July 2023 Gregorian Calendar. Jul 25, 2023 at 1:53
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    @DamionKeeling: I'd guess that only applies to Gentiles; since the Jewish people have celebrated every seventh day as the Sabbath since long before the 4th century. Again, it boils down to "Which calendar?" OP seems to regard dates and calendars as artifacts of nature, when they are in fact always man-made. Jul 25, 2023 at 4:21
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    @cmw: Yes, "construct" is the key word there. But that's the whole crux of the question: just how to perform such a construction, and with what justification? Jul 26, 2023 at 1:04
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    @PieterGeerkens Yes, any calendar is a human invention, I agree. My point wasn't to argue, but to say that pinpointing to a specific day is less artificial, since the day/night grouping is based on natural reactions. That said, it gets trickier when you get into hours/minutes/seconds etc (which are entirely a product of human invention).
    – cmw
    Jul 26, 2023 at 1:17

3 Answers 3


The Gregorian calendar has a leap year cycle of 400 years = 146097 days. Since this happens to be a whole number (20871) of weeks, any date will always be on the same day of the week as its 400th anniversary. Also note that n BCE = -(n-1) CE = 1-n CE, with the "1" being an adjustment for the non-existence of a year zero between 1 BCE and 1 CE.

Putting this together, a calendar for the year n BCE will be identical to the calendar for the year 400m+1-n, where m is any integer.

For example, the calendar for 44 BCE is equivalent to that of 1957 CE, a common year starting on Tuesday, March 15 on a Friday.

Of course, this is for the proleptic Gregorian calendar, not the actual calendar in use at the time. For historical dates, you need to be aware of:

  • When the given location transitioned from the Julian calendar to the Gregorian calendar.
  • The leap year error affecting dates between 44 BCE and 12 CE.
  • The haphazard nature of the Roman calendar before Julius Caesar's reform.
  • The fact that the seven-day week wasn't used (except in Jewish communities) until Christianity became popular.

Also, the date of Easter (and Easter-relative holidays like Mardi Gras) includes a lunar calculation and doesn't repeat every 400 years as the days of the week do. But there'd be no reason for anyone to observe that holiday prior to the crucifixion of Jesus of Nazareth (circa 33 CE), so that's probably not a problem.

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    Despite the flaws involved in correlating with actual dates and practice this seems to be what the question is looking for. Jul 26, 2023 at 2:43
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    If dealing with the Roman civilization, you must also allow for a few days uncertainty between 45 BCE and about 8 CE. The Romans tended to count inclusively, so January 1, 2000 to January 1, 2003 would be counted as 4 years. So on some occasions in the period in question leap years were inserted what we would call 3 years apart, rather than the correct 4 years apart. Augustus corrected this, and finished the correction by 8 CE. The surviving records are not good enough to determine exactly what years were leap years during this period. Jul 26, 2023 at 14:30
  • @GerardAshton: Yes, I already mentioned that in my "leap year error" bullet point.
    – dan04
    Jul 26, 2023 at 15:27

It may actually not be right what you are looking for, i.e. some kind of website on which you can simply do the lookup, but it may help you to understand that the Java programming language has the most sophisticated calender engine I am aware of. You can do quite some sophisticated calculations and convert between different calendars or calculate time spans for example.

Yet you would need to understand a bit of programming. Or maybe it will at least help you to find some kind of web interface to this calendar engine.

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    Jul 26, 2023 at 13:37

For BCE years (and also for CE years <= 1582) you probably want to use Julian, not Gregorian dates. In both cases, the free website you are looking for is Fourmilab's calendar converter.

Note: depending on whether a date in year N BCE is entered as Julian or Gregorian, the year is entered differently:

  • Julian Calendar: -N

  • Gregorian Calendar: -(N-1), with 1 BCE entered as 0

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