This is an interesting detail to learn about history. However, like the article mentions near the end, the Gregorian calendar was adopted at different times in different countries. The Anglican Church didn’t trust the Catholics, so Great Britain adopted the Gregorian calendar in 1752 ( https://en.wikipedia.org/wiki/Calendar_(New_Style)_Act_1750 ). That’s late enough that Benjamin Franklin, George Washington, Thomas Jefferson and others were born before that change.
I think that’s the reason that programming languages tend to adopt the proleptic Gregorian calendar and leave it up to the programmer to figure out when the calendar was adopted in the countries they care about.
Except that countries apparently had to answer various questions when they adopted the Gregorian calendar, and they didn’t all chose the same answers. Ideally, we’d have something like tzdb but for calendar adoption. But I guess the need doesn’t come up all that often.
So, the Proleptic Gregorian Calendar[1] refers to what you'd get if you extended the current Gregorian Calendar back in time before its introduction. (It agrees with the Julian calendar only during the 3rd century AD.) Most date libraries use the Proleptic calendar for simplicity. Ruby's DateTime is unusual insofar as, by default, it actually switches to the Julian calendar for dates before Oct 15 1582.
I'm the author of the Perl DateTime library. It works correctly as _documented_. Here's the first two paragraphs of the docs:
> DateTime is a class for the representation of date/time combinations, and is part of the Perl DateTime project.
> It represents the Gregorian calendar, extended backwards in time before its creation (in 1582). This is sometimes known as the "proleptic Gregorian calendar". In this calendar, the first day of the calendar (the epoch), is the first day of year 1, which corresponds to the date which was (incorrectly) believed to be the birth of Jesus Christ
This is clearly the wrong way to think about it. Dates forward and backward from this flag date exist in both calendars and it's straightforward to convert between them.
Such a flag day didn't involve removing days from one calendar or the other, it switched between them.
It's not a wrong way to think about it. There are some use cases for proleptic calendars, but there are just as many where it's completely inappropriate. If you ever need to compare what day you think it is with what day someone in the past thought it was, then yes those dates don't exist in any real sense.
I wouldn't say it's it's straightforward to convert between them, either. The Gregorian calendar was first adopted in 1582, but the UK and it's colonies didn't follow suit until 1752. Russia didn't convert until 1918, and Greece didn't switch until 1923. We're just over 100 years out from active use of the Julian calendar. So you have to know who the speaker is and where they are or where they are from to know what they mean.
Since these places switched at different times, they had to skip a different number of days. It was 10 in the 16th century. It was 13 in the 20th!
If you're reading a journal by a British man who lived in Russia through the end of the Great War and then moved to Greece in 1920, what does he mean when he's talking about a lunar eclipse he just witnessed?
The way I imagine it is like the calendars are both rulers that are slightly off from each other. We were using one ruler, and at a certain point we went from using one to the other, so our "present" was never mapped to those dates. But you can represent any point in time as any of those calendars, like we can represent a specific temperature as Fahrenheit or Celsius. The calendars extend forwards and backwards to infinity.
There seems to be disagreement about the year. Two systems I checked assume 1752, not 1582.
(OpenBSD)
$ man cal
[...]
The Gregorian Reformation is assumed to have occurred in
1752 after the 2nd of September. By this time, most
countries had recognized the Reformation (although a few
did not recognize it until the early 1900s). Eleven days
following that date were eliminated by the Reformation, so
the calendar for that month is a bit unusual.
(Ubuntu Linux)
[...]
-s country_code
Assume the switch from Julian to Gregorian Calendar at
the date associated with the country_code. If not
specified, ncal tries to guess the switch date from the
local environment or falls back to September 2, 1752.
This was when Great Britain and her colonies switched
to the Gregorian Calendar.
The article is a bit old. It should have noted a new development in calendar technology if it was written half a year later.
> In A.D. 1582 Pope Gregory XIII found that the existing Julian calendar
insufficiently represented reality, and changed the rules about
calculating leap years to account for this. Similarly, in A.D. 2013
Rockchip hardware engineers found that the new Gregorian calendar still
contained flaws, and that the month of November should be counted up to
31 days instead.
> The Eastern Orthodox church even continued well after that, only switching to the Gregorian calendar in 1929.
Technically, they switched to the Revised Julian[0] calendar, not the Gregorian. Also, it was only some of the churches making up Eastern Orthodoxy that switched; others remain on the Julian calendar to this day, and all of them still use the Julian calendar to calculate the date of Pascha.
Fun related fact, many dates -- for example, birth dates on headstones -- of people born in countries observing Eastern Orthodox Christianity will have the prefix O.S. (old style) to disambiguate dates using the original Julian and either the Georgian or Revised Julian calendars which were in effect at that person's death.
A primary reason for the Catholic Church changing the calendar was to bring the date of Easter back into conformance with the decree of the Council of Nicaea on the topic. There was a controversy in the early Church on this, there were even miracles worked concerning the correct date -- see Dom Gueranger's "Liturgical Year" for some of the accounts.
This article presents a grossly simplified version of events. The truth is that only some catholic countries switched in 1582. Britain and its colonies switched in 1752. Prussia switched in 1610. And so on.
In our Dip (in house db) client library, we built what we called a policy that auto selects partitions based on every crud operations. The policies can be specified once for each collection in collections.json based on fields or query.
So for policies where partition involves the date type fields, be it range or fixed dates, it was incredibly hard to generate universal date names from ranges.
The policy mechanism was finished fairly quickly as Dip is inherently distributed first. But reasoning about date names was so incredibly hard.
The missing dates mentioned in the article did come up during development adding to the woes.
But the upside now is that application code no longer worries about the distributed partitions. Policy handles that. Some sort of architecture as code. Don't know whether AAC is an existing term or whether we coined it.
This is (weirdly) in line with behaviour I've observed in iOS's date picker; if you set a block of dates as not available, and the selected date inside of that unavailable block, it will automatically select the first available date on open.
Are we really supposed to trace this given how ancient it is? If we really want to do far we would eventually reach ancient Chinese emperor era names that even confuse historians
tl;dr - ActiveSupport::TimeWithZone used absolute seconds counting to determine dates while DateTime used a proper Gregorian calendar. This means dates before October 15, 1582 have different internal values when compared.
The behavior showed up because a test was using a DateTime value to set a MySQL DATETIME column to the DB minimum value, which is 1000-01-01 00:00:00, and then comparing the DB record attribute to the original variable used to set it.
Something like:
date = DateTime.new(1000, 1, 1, 0, 0, 0, 'UTC')
record.update(happened_at: date)
record.happened_at == date # => false
I "discovered" the missing Gregorian dates when I wrote a loop counting forward in time from Jan 1, 1000 until the corresponding TimeWithZone and DateTime records returned true when compared with ==.
I wonder how many libraries correctly handles the date 1712-02-30 in the Swedish calendar (according to Wikipedia the only official 30th of February in history)?
Tried using ncal like in the examples in that blog post, but it did not work correctly using either Julian or Gregorian. Of course the Swedish calendar from 1700-1712 was neither, but somewhere in-between, because of the (failed) attempt to gradually move from one to the other.
I suppose the Georgian calendar includes this gap officially? Or did the catholic world just jump from system to system? If it includes the gap officially, then yea, error is the right thing. But if we just moved to a new calendar, then arguably there should be no error. both calendars work for all dates continuously, and we could have conversion functions etc. The gap is just a big flag day (week.)
imo, if i am using Georgian i want that system even retroactively, just like projecting a date back into the stone age. I don't care that a particular geography or culture changed calendars.
So I suppose what is really going on is that some libraries are using Georgian consistently, and others are using some hybrid "civic" calendar that tracks what the romans and the catholics were using, so dates in history books make sense.
Contracts and other similar types of financial instruments existed by this time there must be some way they handled it because something that said that it happened on a date that never occurred - sounds like what lawyers live for.
I’ll draft a Dubia so that when future popes change the calendar they will include a representative code sample and details about how we handle this.
(It’s important to remember that the pope and friends were operating much more like an international standards body than a religious leader when doing things like this.)
If a contract dated to the 4th October 1582 specifies you have 15 days to make payment, you would indeed need to know where that contract was signed to determine on what day that deadline expired.
And if there is a contract dated to the 6th October 1582, that does raise questions about its legitimacy (though in general misdating a contract does not void it, it would merely be one point of evidence in your argument that it's a forgery, just like using a particular font in a modern printed contract)
I was thinking of contracts that say "payment is due on 6 October" from before they knew the date of the change.
It's one of those "well technically" things programmers love to obsess over and the judge just says "everyone knows what was meant, stop being an idiot."
Think it depends on usage? Taking October 1st and 20th, and you want to know how many days passed, what's the right answer? If it's days lived, you would want the answer to not be 20, as some of those days didn't happen. But in some other context (perhaps something transitioning to the calendar earlier), they did?
'A date object represents a date (year, month and day) in an idealized calendar, the current Gregorian calendar indefinitely extended in both directions.'
'This matches the definition of the “proleptic Gregorian” calendar in Dershowitz and Reingold’s book Calendrical Calculations.'
Regarding computers and time, I always liked that in AmigaDOS, you could specify custom relative times.
It would natively display "Future" if the file timestamp was ahead of the system clock, but you could also add dates. I remember seeing directory listings like
I think that’s the reason that programming languages tend to adopt the proleptic Gregorian calendar and leave it up to the programmer to figure out when the calendar was adopted in the countries they care about.
Except that countries apparently had to answer various questions when they adopted the Gregorian calendar, and they didn’t all chose the same answers. Ideally, we’d have something like tzdb but for calendar adoption. But I guess the need doesn’t come up all that often.
[1] https://en.wikipedia.org/wiki/Proleptic_Gregorian_calendar
> DateTime is a class for the representation of date/time combinations, and is part of the Perl DateTime project.
> It represents the Gregorian calendar, extended backwards in time before its creation (in 1582). This is sometimes known as the "proleptic Gregorian calendar". In this calendar, the first day of the calendar (the epoch), is the first day of year 1, which corresponds to the date which was (incorrectly) believed to be the birth of Jesus Christ
Always RTFM.
The afternotes at the end of the post are worth reading; people have sent me various qualifications and corrections over the years.
Such a flag day didn't involve removing days from one calendar or the other, it switched between them.
I wouldn't say it's it's straightforward to convert between them, either. The Gregorian calendar was first adopted in 1582, but the UK and it's colonies didn't follow suit until 1752. Russia didn't convert until 1918, and Greece didn't switch until 1923. We're just over 100 years out from active use of the Julian calendar. So you have to know who the speaker is and where they are or where they are from to know what they mean.
Since these places switched at different times, they had to skip a different number of days. It was 10 in the 16th century. It was 13 in the 20th!
If you're reading a journal by a British man who lived in Russia through the end of the Great War and then moved to Greece in 1920, what does he mean when he's talking about a lunar eclipse he just witnessed?
(OpenBSD)
(Ubuntu Linux)> In A.D. 1582 Pope Gregory XIII found that the existing Julian calendar insufficiently represented reality, and changed the rules about calculating leap years to account for this. Similarly, in A.D. 2013 Rockchip hardware engineers found that the new Gregorian calendar still contained flaws, and that the month of November should be counted up to 31 days instead.
https://lwn.net/Articles/669022/
Technically, they switched to the Revised Julian[0] calendar, not the Gregorian. Also, it was only some of the churches making up Eastern Orthodoxy that switched; others remain on the Julian calendar to this day, and all of them still use the Julian calendar to calculate the date of Pascha.
[0] https://en.wikipedia.org/wiki/Revised_Julian_calendar
In our Dip (in house db) client library, we built what we called a policy that auto selects partitions based on every crud operations. The policies can be specified once for each collection in collections.json based on fields or query.
So for policies where partition involves the date type fields, be it range or fixed dates, it was incredibly hard to generate universal date names from ranges.
The policy mechanism was finished fairly quickly as Dip is inherently distributed first. But reasoning about date names was so incredibly hard.
The missing dates mentioned in the article did come up during development adding to the woes.
But the upside now is that application code no longer worries about the distributed partitions. Policy handles that. Some sort of architecture as code. Don't know whether AAC is an existing term or whether we coined it.
tl;dr - ActiveSupport::TimeWithZone used absolute seconds counting to determine dates while DateTime used a proper Gregorian calendar. This means dates before October 15, 1582 have different internal values when compared.
The behavior showed up because a test was using a DateTime value to set a MySQL DATETIME column to the DB minimum value, which is 1000-01-01 00:00:00, and then comparing the DB record attribute to the original variable used to set it.
Something like:
I "discovered" the missing Gregorian dates when I wrote a loop counting forward in time from Jan 1, 1000 until the corresponding TimeWithZone and DateTime records returned true when compared with ==.https://en.wikipedia.org/wiki/List_of_non-standard_dates#Feb...
Tried using ncal like in the examples in that blog post, but it did not work correctly using either Julian or Gregorian. Of course the Swedish calendar from 1700-1712 was neither, but somewhere in-between, because of the (failed) attempt to gradually move from one to the other.
Meta: one can also link to this article, which redirects:
* https://en.wikipedia.org/wiki/February_30
imo, if i am using Georgian i want that system even retroactively, just like projecting a date back into the stone age. I don't care that a particular geography or culture changed calendars.
So I suppose what is really going on is that some libraries are using Georgian consistently, and others are using some hybrid "civic" calendar that tracks what the romans and the catholics were using, so dates in history books make sense.
I’ll draft a Dubia so that when future popes change the calendar they will include a representative code sample and details about how we handle this.
(It’s important to remember that the pope and friends were operating much more like an international standards body than a religious leader when doing things like this.)
And if there is a contract dated to the 6th October 1582, that does raise questions about its legitimacy (though in general misdating a contract does not void it, it would merely be one point of evidence in your argument that it's a forgery, just like using a particular font in a modern printed contract)
It's one of those "well technically" things programmers love to obsess over and the judge just says "everyone knows what was meant, stop being an idiot."
'A date object represents a date (year, month and day) in an idealized calendar, the current Gregorian calendar indefinitely extended in both directions.'
'This matches the definition of the “proleptic Gregorian” calendar in Dershowitz and Reingold’s book Calendrical Calculations.'
https://docs.python.org/3/library/datetime.html#id2 https://docs.python.org/3/library/datetime.html#id5
Whereas Ruby has proleptic Gregorian Time, but civic DateTime:
https://ruby-doc.org/stdlib-2.6.1/libdoc/date/rdoc/DateTime....
It would natively display "Future" if the file timestamp was ahead of the system clock, but you could also add dates. I remember seeing directory listings like