# Julian date calculations and more...

I actually have another question :).

I was working on a calendar system for one of my constructed cultures (world building),and ran into some problems. I’ll have to give some background on it so you can see why i’m asking. It’s set on earth, and the calendar i’m having problems with is the sacred calendar. One sacred day is equivalent to 27 solar days, 9 sacred days equals a month, 27 months equals a year. 162 years equals one cycle (8,736 of our years).

The problem is, I’m having problems figuring out the Gregorian calendar date for 4,368 years ago (we are in the middle of a cycle in the con-world). I need to know it so I can figure out what day, week, and month the culture is in for for the sacred calendar.

Also, what would the Julian day date be for that date as well?

Sorry if this is unclear, but it’s unclear to me as well!
Thanks for any help.

What you’re asking is a trick question for a couple of reasons. A year is not an umambiguously defined unit on a calendar because a solar year is not a round number of days.

The Gregorian calendar is fairly well reconciled to the solar year apart from having to do away with ten whole days when it was implemented. When that ten days was “lost” all depends on what country you live in as it wasn’t universally adopted at the same time. Also there was no Gregorian calendar before 1582 so any date before then is understood to be extrapolation. Your time span also gets into some interesting territory in the leap year cycle.

The Julian calendar as brother Maynard puts it is right out. It’s poorly reconciled to the solar year. Easter was getting all out of what and that’s what put the Pope’s panties in a bunch to begin with.

So when you ask for a date “4,368 years ago” it all depends on what you mean by year.

Ok, So, how are old archaeological sites dated? Do they go by something different? How do they get those dates of 5,980 BCE? Extrapolation? Guessing?

Not sure that this will answer your question, but what the hell:

If you know a Gregorian calendar date, here’s a formula that will tell you what day of the week it falls on:

W = k + [2.6m - .2] - 2C + Y + [Y/4] +[C/4]

Reduce W mod 7 (the least positive remainder of W when you divide it by 7). The days Sun., Mon.,…,Sat. correspond to W=0,1,…,6, respectively.

k = numerical date of the month

m = month (March, April,…, Feb. correspond to m = 1,2,…,12, respectively)

C = all but the last two digits of the year (C=19 now)

Y = last two digits of the year

= x rounded down to the nearest integer

So, for example, today, k=30, m=9, C=19, Y=99, and we get

W = 30 + [23.2] - 38 + 99 + [24.75] + [4.75]

= 142,

which has remainder 2 when you divide by 7, so today is Tuesday (as if you didn’t know that already).

This will work for any date of the Gregorian calendar, including if you extrapolate the Gregorian calendar back to before it was invented. Anyway, probably not the answer to your question, but maybe it will help some.

Well, one thing to consider when messing with dates is that there are three separate “Julian” calculations.

The easiest is the Julian Date known and loved by all COBOL programmers in financial systems. This is simply the three-digit “day of the year” following the two- or four-digit year. (It is often found at the bottom of the page on “Day Calendars” and tells you the number of days since the last December 31.) It is used to calculate accounting periods, Accounts Receivables aging, etc. January 1, 1999 is 99001. March 1, 1999 is 99060. March 1, 2000 is 00061 (leap year having added a day).

You really don’t care about Julian Date.

The next Julian is the Julian Calendar. This is the reformed calendar 365.25 day year that Julius Caesar had commissioned from some Egyptian astronomers to get a year that had some semblance of regularity in regards to the earth’s orbit of the sun. Caesar Augustus had it fine-tuned around 8 CE and it worked pretty well until Pope Gregory noticed that the year was out of synch with the sun by about 10 days–at which time he ordered his reform, giving us the Gregorian Calendar.

You don’t have any need for the Julian Calendar, either.
Finally, there is the Julian Period. This is a method of reckoning time in days, beginning on January 1, 4713 BCE. It was created by Joseph Scaliger, in France in 1582 (and named after his father, Julius). (That year is also the year Gregory XIII ordered his new calendar.)

The beginning date was picked as the most recent synchronization of the cycles of the 28-year solar cycle (after which the days of the year match the days of the week), the 19-year lunar cycle (after which the phases of the moon occur on the same days in the year), and the 15-year indiction cycle used for tax calculations in ancient Rome. There is a short piece on this that I cribbed from the World Almanac and Book of Facts©, but you could probably find some more info with a web search for “julian period.”

This is probably the Julian somethingorother you need.

(Since all calendars have days, dates, and periods, the terms Julian Date, Julian Calendar, and Julian Period are used to refer to each of the preceding in different contexts–just to make it confusing.)

There are many web sites that have date conversion routines in C, C++, and other languages.

You should probably look up this site:
http://www.panix.com/~wlinden/calendar.shtml

Tom~

Oops, not that anyone really cares, but I forgot to mention that, in the above formula, if you’re using it to calculate a day of the week in Jan. or Feb., you consider those months part of the previous year. In other words, for example: next year, for Jan. and Feb. we still have C and Y equal 19 and 99, respectively, instead of 20 and 00.

As for the archaeological sites: if the date was acheived by something like radio carbon dating it will not be particularly precise-- like there’s no way they can know a decade specifically, and the number they give it to is to some degree arbitrary. Otherwise, if there is historical evidence, they will cross reference with the dates of neighboring cultures (ie, the Greeks say it’s X year, and Egypt records the same event in Y year, which we know by our calendar was Z, roughly (I assume the relative stability of the Roman calendar in the BC to AD jump we have in our calendar helps out here).

I know that, for example, in the Mathura dynasty in Kushan era in part of India certain events took place which we are provided their dates for, but for some reason no other culture contemporary to them records their activities so we have no way to attach their internal dating to any other system, so they best we can do is centuries when discussing that history.
I have seen historical/archaeological systems where the date of something is given as “B.P.”-- before the present date.
I’d say just ballpark it to within the nearest decade (that should be possible) and let it go at that and the other players will never know the difference. But I’m sloppy in general.