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Comment by kccqzy | Hacker News Reader

I wrote my own date calculation functions a while ago. And during that, I had an aha moment to treat March 1 as the beginning of the year during internal calculations[0]. I thought it was a stroke of genius. It turns out this article says that’s the traditional way.

[0]: https://github.com/kccqzy/smartcal/blob/9cfddf7e85c2c65aa6de...

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not completely coincidentally, March was also the first month of the year in many historical calendars. Afaik that also explains why the month names have offset to them (sept, oct, nov, dec)

edit: I just love that there are like 5 different comments pointing out this same thing

I've read that not only March was the first month, but the number of months was only ten: winter months did not need to be counted because there was no agricultural work to be done (which was the primary purpose of the calendar). So after the tenth month there was a strange unmapped period.

>So after the tenth month there was a strange unmapped period.

this is when time-travelling fugitives hide out

How do you figure out it's March 1 if you're not counting days?

Equinox or something like that?

The precise equinox sounds fussy to measure and even then you need to know three weeks before the equinox. While counting days is very easy.

Yes. But they also added leap days on an ad hoc basis right until the Caesar reformed the calendar. So some fussiness would probably not deter the much earlier Romans.

(It's still evidence in the direction you suggest, just much weaker than it looks at first.)

Druid tells you

> explains why the month names have offset to them (sept, oct, nov, dec)

Everything now makes sense, I always wondered why September was the nine month with a 7 prefix.

I thought Sept, Oct, Nov, and Dec were shifted by the addition of July (Julius) and August (Augustus)?

That's a common misconception. Those were just renamed for the Caesars. January and February we added, before that there was just a gap in the winter.

TIL thanks!

What were they before? Quintember? Sextober?

Basically, yes. Quintilis and Sextilis.

> not completely coincidentally, March was also the first month of the year in many historical calendars.

And often the last month too. The early modern English calendar began the year on March 25.

This is coincidental in relation to the offset in the names of the months. The Romans started their year in January just like we do today.

(Though in a very broad sense, it's common to begin the year with the new spring. That's the timing of Chinese new year and Persian new year. I believe I've read that the Roman shift two months backward was an administrative reform so that the consuls for the year would have time to prepare for the year's upcoming military campaigns before it was time to march off to war.)

Also: March is named after the Roman god of war, Mars.

This is because March is when they would begin to mobilize armies for campaigns. The timing is chosen by when winter wheat will be ready for harvest, so soldiers will have nearby high-calorie foot to pilfer while on campaign.

One tricky part of pre-industrial armying is that you can mostly only bring what food you can carry. Things that carry food (e.g. donkeys) require food themselves. So then you have to bring feed as well, which requires more donkeys… etc.

Instead, they would “forage” local areas. If they got there too soon, there is nothing en masse to take!

The first article in this blog post series has a little section talking briefly about this history, and there's a representation of this that I think sheds a lot of light on the original design. See the heading "Side-Note on Month / Day Determination" in the below link [1].

Displaying the months like the following helps see the regularity at a glance. Columns 1, 3 and 5 are the long months, others being shorter:

  +-----+-----+-----+-----+-----+
  | 31  | 30  | 31  | 30  | 31  |
  |  I  | II  | III | IV  |  V  |
  | MAR | APR | MAY | JUN | JUL |
  |-----+-----+-----+-----+-----+
  | 31  | 30  | 31  | 30  | 31  |
  | VI  |VII  |VIII | IX  |  X  |
  | AUG | SEP | OCT | NOV | DEC |
  +-----+-----+-----+-----+-----+
  | 31  |28/29|
  | XI  |XII  |
  | Jan | FEB |
  +-----+-----+

> To a person who natively thinks in Roman numerals, remembering that the short months are: II, VII, XII, along with IV & IX would be much easier than the way us modern folks have to memorise it.

[1] https://www.benjoffe.com/fast-date

At this risk of me feeling stupid, could you briefly explain the benefit of this?

It's easy to know what day of the year it is because leap days are at the end.

I just added a link to the code with a brief comment. Basically, it simplifies the leap year date calculation. If February is the last month of the year, then the possibly-existing leap day is the last day of the year. If you do it the normal way your calculations for March through December need to know whether February is a leap year. Now none of that is needed. You don’t even need explicit code to calculate whether a given year is a leap year: it’s implicit in the constants 146097, 36524, and 1461.

The magic numbers at the end of this explanation are the number of days of each part of the leap year cycle:

146097 days = 400 year portion of the leap year cycles (including leap years during that)

36524 days = same for the 100 year portion of the leap year cycles

1461 days = 4 year cycle + 1 leap day

IIRC, it's also why the leap day was set to Feb 29th in the first place. At the time (romans?) the year started March 1st.

In case someone was wondering why in the world someone said we should add a day to the second month of the year...

And (oct)ober was the 8th month of the year, (nov)ember the ninth, (dec)ember the tenth!

Don't forget (sep)tember being the 7th month

Weird parenthesization. The latin numbers are septem, octo, novem, decem for 7, 8, 9, 10. And then they all have a -ber suffix.

The three letter prefixes show up in English (eg oct in octal, dec in decimal, etc.).

The prefix in "decimal" is "decim", like you'd expect given that the root is "decem". There is no "dec-" prefix.

(You do see "deca" used as a prefix, "a" included, but that doesn't come from Latin.)

You are right. I should have written: (septem)ber was the 7th month of the year, (octo)ber was the 8th, (novem)ber the ninth, and (decem)ber the tenth!

That's correct, the Romans had March as the first month of the year, so leap day was the last day of the year and September, October, November and December were the 7th (sept), 8th (oct), ninth (nov) and 10th (dec) months.

June and July used to be Quintilis and Sextilis.

I think Quintilis and Sextilis were renamed to July and August, in honor of Julius and Augustus, respectively.

The calendar was regularized to include a leap day during the reign of Julius Caesar (hence the name "Julian calendar"), which would have been 45 BC.

The Roman calendar moved to January as the first month of the year in 153 BC, over a hundred years before the leap day was added. The 10-month calendar may not have even existed--we see no contemporary evidence of its existence, only reports of its existence from centuries hence and the change there is attributed to a mythical character.

Are you saying that while we do see evidence that September, October, November, December were once the 7th, 8th, 9th, and 10th month, we don't see any evidence that the calendar was ever "10 months long"? (How would that have worked anyway, did they have more days per month?)

Pretty much.

> How would that have worked anyway, did they have more days per month?

The way I've heard, they just simply didn't track the date during the winter.

Wikipedia gives 3 dates for January being the first month, either (approx) 700, 450, 150 BCE.

It's fair to say January was the first month of the Roman calendar; despite it having formerly been March.

Btw, the Romans had leap days before Julius Caesar, but they were added ad hoc by the Pontifex Maximus.

Caesar happened to be the Pontifex Maximus (an office you hold for life once elected to), but he wasn't in Rome much to do that job. So after he came back from hanging out with Cleopatra in Egypt he came back and set the calendar on auto-pilot.

I don't know if it ever made it to production, and I don't remember exactly why it made sense at the time, but one early hack I did was passing a date in Julian format because there weren't enough bits to pass a full timestamp.

> The Roman calendar moved to January as the first month of the year in 153 BC, over a hundred years before the leap day was added.

Note that some sources suggest that February might still have been the last month, placed after December and before January.

Technically, the leap day (bissextus) was the 24th. (Wikipedia tells me this is because that's when Mercedonius used to be, before the Julian reforms.)

Not so relevant, but some fun history, the Roman calendar did start in March, so tacking on the leap years was done at the finale. This also meant that the root of the words - the "oct" in october means 8 was also the eighth month of the year.

As well as the leap year stuff people have mentioned, there was something else that I've got a vague memory of (from an old SciAm article, IIRC, which was about using March as the first month for calculations) which pointed out that if you use March as 0, you can multiple the month number by (I forget exactly what but it was around 30.4ish?) and, if you round the fraction up, you get the day number of the start of that month and it all works out correctly for the right 31-30-31 etc sequence.

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