Per Jc3s5h's comments at T95553, apparently the way that Wikidata stores and displays datetime data doesn't cleanly map to centuries and millenniums. Ghouston has suggested using "20XX" or "2000-2099" instead of "X. century". This is also related to T73459 (and might be considered a duplicate of that).
|Open||None||T87764 Bugs related to time datatype (tracking)|
|Open||None||T196674 Stop using "century" and "millennium" in association with Wikidata datetime data|
For the record, I don't think this is a good solution and I support continued use of "century" and "millennium" as they are the most intuitive ways for editors to enter this information. It's also the way that date information is recorded on Commons and it will be important that Wikidata support that for the Structured Data on Commons project. Perhaps Wikidata just needs to change how it parses these dates (i.e. T73459).
The actual bug is that some software (such as the Wikidata user interface) doesn't seem to be following the definition of the "precision" field in a date.
Precision, as described at https://www.wikidata.org/wiki/Help:Dates, specifies how many digits of the date string should be ignored.
E.g., +2018-06-00T00:00:00Z is not a valid date, but with precision 10 (month) it specifies the range 2018-06-01 to 2018-06-30.
With precision 7 (century) it specifies the range 2000-01-01 to 2099-12-31. When this date is entered into a Wikidata field and precision set to 7, it displays "21 .century". This would be OK if it was generally accepted that Wikidata takes the century as 2000-2099, and not 2001-2100 as is sometimes used.
However, if the date +2000-00-00T00:00:00Z if entered with precision 7, the user interface displays it as "20 .century", which is now confusing everybody. I suggested avoiding the word "century" entirely.
Current Wikibase definition of beginning and ending years of a century and millennium are in synch with the definitions in the English or German Wikipedia, see :de:19._Jahrhundert for example. Maybe Wikibase software could just provide a link to the item like Q6955 to clarify the definition, and where we can specify the date range.
Wikibase is not a single thing. It is made up of many parts, and currently the user interface disagrees with all the other parts about what range is indicated when the precision is set to 7.
The change in the Dates help page was made by Jarekt at 14:28, 17 July 2018 UTC. The edit summary states
Switched text from what "should" be the first and last year of decade century or millennium, to what is currently interpreted by the wikibase software
I don't know about this edit, but other editors have been known to claim the wikibase software works a certain way, when what they really investigated was merely the user interface.