I may have said this before; it is very easy to get things screwed up when a value must reference a datum (a calendar is a datum for time). I think this is perhaps one of the most common errors on Wikipedia, we just assume there is a single global datum. Usually it is not, it is only a matter of precission in the number and some datum-error bites you in the rumpf!

Big thanks to Lydia and the team for providing an explanation, it makes it much easier to fix it. Don't waste to much time on how it happen, it is far more important to figure out how to fix it.

Keep up the good work, and don't forget to report failures. Thats how we all learn.

John Erling Blad
/jeblad

(Okey I do laugh a little, but not very much! )

On Tue, Jun 30, 2015 at 9:21 PM, Joe Filceolaire <filceolaire@gmail.com> wrote:

Can I just ask all of you who want to demand an enquiry as to how this happened to hold off until the problem has been fixed

Please

No post mortem while the patient is still alive

Joe


On Tue, 30 Jun 2015 18:39 Lydia Pintscher <lydia.pintscher@wikimedia.de> wrote:
Hi everyone,

I have some bad news. We screwed up. I’m really sorry about this. I’d
really appreciate everyone’s help with fixing it.

TLDR: We have a bad mixup of calendar models for the dates in Wikidata
and we need to fix them.

==== What happened? ====
Wikidata dates have a calendar model. This can be Julian or Gregorian
and the plan is to support more in the future. There are two ways to
interpret this calendar model:
# the given date is in this calendar model
# the given date is Gregorian and this calendar model says if the date
should be displayed in Gregorian or Julian in the user interface

Unfortunately both among the developers as well as bot operators there
was confusion about which of those is to be used. This lead to
inconsistencies in the backend/frontend code as well as different bot
authors treating the calendar model differently. In addition the user
interface had problematic defaults. We now have a number of dates with
a potentially wrong calendar model. The biggest issue started when we
moved code from the frontend to the backend in Mid 2014 in order to
improve performance. Prior to the move, the user interface used to
make the conversion from one model to the other. After the move, the
conversion was not done anywhere anymore - but the calendar model was
still displayed. We made one part better but in the process broke
another part badly :(

==== What now? ====
* Going forward the date data value will be given in both the
normalized proleptic Gregorian calendar as well as in the calendar
model explicitly given (which currently supports, as said, proleptic
Gregorian and proleptic Julian).
* The user interface will again indicate which calendar model the date
is given in. We will improve documentation around this to make sure
there is no confusion from now on.
* We made a flowchart to help decide what the correct calendar model
for a date should be to help with the clean up.
* We are improving the user interface to make it easier to understand
what is going on and by default do the right thing.
* We are providing a list of dates that need to be checked and
potentially fixed.
* How are we making sure it doesn’t happen again?
* We are improving documentation around dates and will look for other
potential ambiguous concepts we have.

==== How can we fix it? ====
We have created a list of all dates that potentially need checking. We
can either provide this as a list on some wiki page or run a bot to
add “instance of: date needing calendar model check“ or something
similar as a qualifier to the respective dates. What do you prefer?
The list probably contains dates we can batch-change or approve but
we’d need your help with figuring out which those are.
We also created a flowchart that should help with making the decision
which calendar model to pick for a given date:
https://commons.wikimedia.org/wiki/File:Wikidata_Calendar_Model_Decision_Tree.svg

Thank you to everyone who helped us investigate and get to the bottom
of the issue. Sorry again this has happened and is causing work. I
feel miserable about this and if there is anything more we can do to
help with the cleanup please do let me know.


Let's please keep further discussion about this in one place on-wiki
at https://www.wikidata.org/wiki/Wikidata:Project_chat#calendar_model_screwup


Cheers
Lydia

--
Lydia Pintscher - http://about.me/lydia.pintscher
Product Manager for Wikidata

Wikimedia Deutschland e.V.
Tempelhofer Ufer 23-24
10963 Berlin
www.wikimedia.de

Wikimedia Deutschland - Gesellschaft zur Förderung Freien Wissens e. V.

Eingetragen im Vereinsregister des Amtsgerichts Berlin-Charlottenburg
unter der Nummer 23855 Nz. Als gemeinnützig anerkannt durch das
Finanzamt für Körperschaften I Berlin, Steuernummer 27/681/51985.

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