# When does historical chronology break down? **URL:** \ **Category:** Factual Questions\ **Created:** [April 12, 2023, 1:57pm UTC](https://boards.straightdope.com/t/when-does-historical-chronology-break-down/982580 "2023-04-12T13:57:50Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![brutus44](https://avatars.discourse-cdn.com/v4/letter/b/d9b06d/32.png) [@brutus44](https://boards.straightdope.com/u/brutus44)\ **Post date:** [April 12, 2023, 1:57pm UTC](https://boards.straightdope.com/t/when-does-historical-chronology-break-down/982580/1 "2023-04-12T13:57:51Z")
If something happened 10 or 20 years ago, I can easily do some simple math and find out exactly how many days ago it was. I assume the same applies for events in the 20th century. However, can this be applied to dates that are really far back in the past? I know leap days were invented sometime in the middle ages, and I read somewhere that a few months had to be deleted out of the calendar to “fix” it some centuries ago. A related question: When scientists reconstruct ancient eclipses, how do they assign them dates? Is there a “modern calendar” that can be overlaid over all the different ones ancient societies used? ---
**Author:** ![LSLGuy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lslguy/32/5813_2.png) [@LSLGuy](https://boards.straightdope.com/u/LSLGuy)\ **Post date:** [April 12, 2023, 2:12pm UTC](https://boards.straightdope.com/t/when-does-historical-chronology-break-down/982580/2 "2023-04-12T14:12:13Z")
That is a very big topic. See here > **[History of calendars](https://en.wikipedia.org/wiki/History_of_calendars)** > > The history of calendars, that is, of people creating and using methods for keeping track of days and larger divisions of time, covers a practice with ancient roots. > Archeologists have reconstructed methods of timekeeping that go back to prehistoric times at least as old as the Neolithic. The natural units for timekeeping used by most historical societies are the day, the solar year and the lunation. Calendars are explicit schemes used for timekeeping. The first historically attested and formu... then get back to us. The current dating system is simple and easy to use for most of the world. Although it was formulated in the late 1500s it took almost 200 years until the mid 1700s for it to become almost universal. So for those ~150 years in order to compute or convert a date you needed to know where on earth you were talking about, not just when. And even after teh mid 1700s there were holdout countries refusing to use it. Before the 1500s reform it was a gigantic mess, and not a fully-documented mess. Current scholarship has some consensus on agreed upon conversions from one to another. They may not be historically accurate to the exact day, but at least they’re currently agreed upon so any two people doing a conversion today of a date 1200 years ago into e.g. the 11th century Persian calendar will get the same answer. Some software handles most or all of this correctly, and some in effect assumes the current calendar extends backwards forever. Unless you know the limitations of your particular software, you can’t reliably apply it to stuff much before 1750. Whether you care about that level of accuracy and provable reliability depends on what you’re trying to do with the result. ---
**Author:** ![brutus44](https://avatars.discourse-cdn.com/v4/letter/b/d9b06d/32.png) [@brutus44](https://boards.straightdope.com/u/brutus44)\ **Post date:** [April 12, 2023, 2:19pm UTC](https://boards.straightdope.com/t/when-does-historical-chronology-break-down/982580/3 "2023-04-12T14:19:01Z")
Thanks for the quick answer, much appreciated. So do historians today have a “lingua franca” for calendars that they use to convert between them? Say a researcher wants to study multiple eurasian societies in the 2nd century CE, does he just convert all the dates to the current calendar extended backwards? > [@LSLGuy](#): > > They may not be historically accurate to the exact day Why not? Aren’t there common dates that can be used to link any 2 calendars to each other? I would assume it would be relatively straightforward to work backwards from there. ---
**Author:** ![LSLGuy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lslguy/32/5813_2.png) [@LSLGuy](https://boards.straightdope.com/u/LSLGuy)\ **Post date:** [April 12, 2023, 2:24pm UTC](https://boards.straightdope.com/t/when-does-historical-chronology-break-down/982580/4 "2023-04-12T14:24:33Z")
Nobody knows _exactly_ which day the 34th of Frumbblebum in Mesopotamia in roughly 3500 BC was. Nobody back then thought to keep an exact list of conversions between then and every calendar since. And even if they had, at least one of those cheat sheets would have been lost to antiquity 2,000 years ago. Based on whatever fragmentary evidence may exist today, modern historians can make educated guesses and estimates. But there’s no way we can be certain today that our estimates are perfect. There simply is no data available to prove one way or another. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [April 12, 2023, 2:35pm UTC](https://boards.straightdope.com/t/when-does-historical-chronology-break-down/982580/5 "2023-04-12T14:35:24Z")
> [@LSLGuy](#): > > Nobody knows _exactly_ which day the 34th of Frumbblebum in Mesopotamia in roughly 3500 BC was. In many cases, we do, because we have astronomical observations recorded in that calendar. And it is of course possible to describe a point in time using any of a variety of modern calendar systems, because the modern systems are all well-specified. In some cases, it might be necessary to explicitly state which calendar system is being used: If, for instance, one refers to “October 14, 1066”, then one should specify whether one means according to the Gregorian calendar (which is used today), or the Julian calendar (which was what was used at the time). Other calendar systems are also used, some of which are much easier to work with. For instance, astronomers often use something called the “Julian day” (which is not related to the old Julian calendar), which is simply a continual count of days since a certain reference date far in the past (for instance, right this moment, it’s 2460047.1067). To find the time elapsed between two moments of time, one need merely subtract the Julian day numbers, without needing to worry about how many days are in each month or the like (in fact, a computer calculating time elapsed is likely to do so by first converting both dates to Julian day or something similar). ---
**Author:** ![brutus44](https://avatars.discourse-cdn.com/v4/letter/b/d9b06d/32.png) [@brutus44](https://boards.straightdope.com/u/brutus44)\ **Post date:** [April 12, 2023, 2:36pm UTC](https://boards.straightdope.com/t/when-does-historical-chronology-break-down/982580/6 "2023-04-12T14:36:17Z")
This makes sense, but I’m still very confused, so bear with me if you can. Lets take a random example: Wikipedia says the battle of the Teutoburg forest took place in September, 9 AD. Is this the according to the current calendar? Can I say that it was exactly 2013 years before 2022 and therefore 2013\*365.25 days before September 2022? And how do historians even know this? As you just said, it’s all just estimates when we’re looking this back in time, so how we be certain? Should there be a error range when giving dates in antiquity? Should textbooks say “Caesar was assassinated in 44 BC, ±6 months” instead of stating the exact date like it’s known mathematically? It seems to me that there’s a growing fuzziness in dating from about 1500 that grows as you head backwards in time, and it’s impossible to date most things with certainty beyond that point. I could be very wrong though! ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [April 12, 2023, 2:42pm UTC](https://boards.straightdope.com/t/when-does-historical-chronology-break-down/982580/7 "2023-04-12T14:42:25Z")
There isn’t a cutoff at which this happens. The Romans kept excellent records, and so we know definitely from those records that Caesar was assassinated on the date called, by their calendar, the Ides of March. And there are enough places that have kept good records since then, that we can reliably relate that date in the Roman calendar system to a date in our modern calendar system. On the other hand, there have been some places and times in history, and some places even now, where record-keeping isn’t so good, and so we can only estimate when certain events were. In some parts of Africa, for instance, children don’t start school when they are at least 5 years old by a certain date (as is usual in the US), because records of birthdates are spotty, and they instead start school when they look like they’re about old enough (the usual criterion is “when a child can reach over the top of their head and touch the opposite ear”). ---
**Author:** ![Thudlow\_Boink](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/thudlow_boink/32/320_2.png) [@Thudlow\_Boink](https://boards.straightdope.com/u/Thudlow_Boink)\ **Post date:** [April 12, 2023, 2:48pm UTC](https://boards.straightdope.com/t/when-does-historical-chronology-break-down/982580/8 "2023-04-12T14:48:29Z")
> [@brutus44](#): > > I read somewhere that a few months had to be deleted out of the calendar to “fix” it some centuries ago. If you’re referring to the changeover from the Julian to the Gregorian calendar, it was 10 days that had to be skipped. > **[From Julian to Gregorian Calendar](https://www.timeanddate.com/calendar/julian-gregorian-switch.html)** > > The Gregorian calendar was introduced in 1582 in some countries. The US, Canada, and the UK changed in 1752. Why were some days skipped? ---
**Author:** ![zimaane](https://avatars.discourse-cdn.com/v4/letter/z/a3d4f5/32.png) [@zimaane](https://boards.straightdope.com/u/zimaane)\ **Post date:** [April 12, 2023, 3:41pm UTC](https://boards.straightdope.com/t/when-does-historical-chronology-break-down/982580/9 "2023-04-12T15:41:41Z")
The Romans even minted a coin commemorating the fact that the assassination took place on the Ides of March: > **[Ides of March coin](https://en.wikipedia.org/wiki/Ides_of_March_coin)** > > The Ides of March coin, also known as the Denarius of Brutus or the EID MAR, is a rare version of the denarius coin issued by Marcus Junius Brutus from 43 to 42 BC. The coin was struck to celebrate the March 15, 44 BC, assassination of Julius Caesar. It features a bust of Brutus, who was one of the assassins, on the obverse while the reverse features a pileus cap between two daggers. The coin was minted in both silver and gold. Approximately 100 of the silver coins are known to exist, but only t... ---
**Author:** ![Lumpy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lumpy/32/446_2.png) [@Lumpy](https://boards.straightdope.com/u/Lumpy)\ **Post date:** [April 12, 2023, 4:23pm UTC](https://boards.straightdope.com/t/when-does-historical-chronology-break-down/982580/10 "2023-04-12T16:23:21Z")
Incidentally there’s a woo theory that several centuries were simply invented out of whole cloth: [Phantom time hypothesis - Wikipedia](https://en.wikipedia.org/wiki/Phantom_time_hypothesis) Needless to say, there’s tons of hard evidence that this simply couldn’t have happened. ---
**Author:** ![Saint\_Cad](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/saint_cad/32/18907_2.png) [@Saint\_Cad](https://boards.straightdope.com/u/Saint_Cad)\ **Post date:** [April 12, 2023, 4:49pm UTC](https://boards.straightdope.com/t/when-does-historical-chronology-break-down/982580/11 "2023-04-12T16:49:03Z")
> [@Thudlow\_Boink](#): > > If you’re referring to the changeover from the Julian to the Gregorian calendar, it was 10 days that had to be skipped. 11 days for those countries that converted later like the Soviet Union. ---
**Author:** ![Kimstu](https://avatars.discourse-cdn.com/v4/letter/k/ecd19e/32.png) [@Kimstu](https://boards.straightdope.com/u/Kimstu)\ **Post date:** [April 12, 2023, 6:04pm UTC](https://boards.straightdope.com/t/when-does-historical-chronology-break-down/982580/12 "2023-04-12T18:04:41Z")
> [@Chronos](#): > > In many cases, we do, because we have astronomical observations recorded in that calendar. Nitpick: Although often such observational records are imprecise enough that we can’t use astrochronological means to pinpoint the event unambiguously. Often what we end up with is just a narrowed-down set of possible dates (e.g., “it could be referring to any of these three lunar eclipses within this ten-year period”). ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [April 12, 2023, 6:30pm UTC](https://boards.straightdope.com/t/when-does-historical-chronology-break-down/982580/13 "2023-04-12T18:30:29Z")
Sometimes, yes, if all you have to go on is an eclipse of the Moon. But some ancient civilizations kept records of multiple celestial objects as well, such as Venus. They end up with enough information that the only possible dates to match the observations are thousands of years apart, far enough that other dating techniques are more than adequate to distinguish between them. ---
**Author:** ![JohnT](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/johnt/32/15048_2.png) [@JohnT](https://boards.straightdope.com/u/JohnT)\ **Post date:** [April 12, 2023, 6:41pm UTC](https://boards.straightdope.com/t/when-does-historical-chronology-break-down/982580/14 "2023-04-12T18:41:42Z")
> [@brutus44](#): > > Should textbooks say “Caesar was assassinated in 44 BC, ±6 months” instead of stating the exact date like it’s known mathematically? Welcome to the Dope, Brutus! Pretty sure the statute of limitations have passed on that whole Caesar thing, so you should be safe. 😉 ---
**Author:** ![bump](https://avatars.discourse-cdn.com/v4/letter/b/7c8e57/32.png) [@bump](https://boards.straightdope.com/u/bump)\ **Post date:** [April 12, 2023, 7:35pm UTC](https://boards.straightdope.com/t/when-does-historical-chronology-break-down/982580/15 "2023-04-12T19:35:57Z")
> [@Chronos](#): > > And there are enough places that have kept good records since then, that we can reliably relate that date in the Roman calendar system to a date in our modern calendar system. I kind of think that’s what the OP’s getting at- how do we know that the Ides of March in 710 AUC is actually March 15, 44 BC by our calendar? My suspicion is that enough of the old calendars ended up overlapping the modern ones that for example, we know that today is April 12, 2023 on the Gregorian Calendar, March 30, 2023 by the Julian Calendar, 21 Nisan 5783 by the Hebrew Calendar, and AUC MMDCCLXXVI by the ab urbe condita calendar. I think the fuzziness really kicks in somewhere a little before the AD/BC change; the Julian calendar was adopted by the Romans in 46 BC, so it’s been around a while. And the Church was essentially the timekeeper for a LONG time as well, so it’s more or less in the pre-Julian, pre-Church eras where there might be some fuzziness. But even there, I suspect there have been enough astronomical events like eclipses that are corroborated in writings to still pin things down pretty well. ---
**Author:** ![dtilque](https://avatars.discourse-cdn.com/v4/letter/d/d6d6ee/32.png) [@dtilque](https://boards.straightdope.com/u/dtilque)\ **Post date:** [April 12, 2023, 10:16pm UTC](https://boards.straightdope.com/t/when-does-historical-chronology-break-down/982580/16 "2023-04-12T22:16:53Z")
> [@Saint\_Cad](#): > > > [@Thudlow\_Boink](#): > > > > If you’re referring to the changeover from the Julian to the Gregorian calendar, it was 10 days that had to be skipped. > > 11 days for those countries that converted later like the Soviet Union. It was 11 days when Great Britain changed in the 18th century. By the 20th century, when the Soviet Union (also Bulgaria and Greece) converted, it was 13 days. ---
**Author:** ![md-2000](https://avatars.discourse-cdn.com/v4/letter/m/9d8465/32.png) [@md-2000](https://boards.straightdope.com/u/md-2000)\ **Post date:** [April 12, 2023, 10:26pm UTC](https://boards.straightdope.com/t/when-does-historical-chronology-break-down/982580/17 "2023-04-12T22:26:53Z")
> [@dtilque](#): > > It was 11 days when Great Britain changed in the 18th century. By the 20th century, when the Soviet Union (also Bulgaria and Greece) converted, it was 13 days. Fun trivia question - Q: what month was the October Revolution? A: November. IIRC the Julian calendar replaced a system where the Romans would simply add arbitrary days occasionally every few years to get the calendar back in sync with the seasons. So perhaps back then, dates get a little murky. But any calendar predictable and reliable that overlapped at some time with the Julian calendar is probably reliable too. (Egypt? Greece?) It seems to me that perhaps nation-wide catastrophes might throw local calendars off, dependin on how reliably they were calibrated to astronomical events, because ultimately that’s what the calenders were tied to. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [April 13, 2023, 1:15pm UTC](https://boards.straightdope.com/t/when-does-historical-chronology-break-down/982580/18 "2023-04-13T13:15:15Z")
Another fun one is that the greatest authors in the English and Spanish language both have the same birthdate, but Cervantes was born a week and a half before Shakespeare. ---
**Author:** ![Steve\_MB](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/steve_mb/32/5339_2.png) [@Steve\_MB](https://boards.straightdope.com/u/Steve_MB)\ **Post date:** [April 13, 2023, 1:25pm UTC](https://boards.straightdope.com/t/when-does-historical-chronology-break-down/982580/19 "2023-04-13T13:25:43Z")
> [@md-2000](#): > > IIRC the Julian calendar replaced a system where the Romans would simply add arbitrary days occasionally every few years to get the calendar back in sync with the seasons. So perhaps back then, dates get a little murky. My recollection is that this system got increasingly corrupted in the late Republic (days would be added or not based on who got more or less time in power until the next elections). ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [April 13, 2023, 1:36pm UTC](https://boards.straightdope.com/t/when-does-historical-chronology-break-down/982580/20 "2023-04-13T13:36:16Z")
Even at that, I think that the Romans still generally left records of their calendar shenanigans. You can do pretty much whatever you want with the calendar and get away with it, provided you left records of it (as long as those particular records weren’t lost at some point). [Next page](https://boards.straightdope.com/t/when-does-historical-chronology-break-down/982580.md?page=2)