Dates in any format except ISO 8601 (that is, like 1582-10-04) are normally understood to be in either the Julian or Gregorian calendar, whichever was in effect at the place and time of interest. The ISO 8601 standard specifies that those dates are always in the Gregorian calendar or proleptic Gregorian calendar (that is, Gregorian rules extended backward before that calendar existed). Conversion from one format to the other causes a change in the meaning of the date. There is no warning on the preferences date and time menu (at least in English) that the software has no metadata about the calendar, and has no ability to convert Julian to/from Gregorian.

Simple solution: Eliminate ISO dates as a display option.

More nuanced solution: divide the preference between

(1)general purpose dates (for which no ISO display option is offered) and

(2)timestamps for events that occur within the servers running wikimedia software (which are almost certainly in the Gregorian calendar) for which the ISO display format is offered.

Inadequate workaround: warn on the date and time preference window that readers who choose the "2001-01-15T16:12:34" should not read any text discussing any person or event before AD 1752.

Version: unspecified
Severity: normal