RangeCheckerHelper (used by RangeChecker and DiffWithinRangeChecker, “range” and “difference within range” constraint types) uses TimeValueCalculator to operate on dates, which throws away the calendar of a TimeValue and assumes that its time stamp is in the Gregorian calendar. We need to fix that at some point. Wikibase already seems to have some provision to convert Julian dates to Gregorian ones for the RDF export, so I assume we can reuse that.