Now the only calendar for inputting data in Wikidata is the Gregorian calendar. there are some other local calendars which are supported by MediaWiki at [https://www.mediawiki.org/wiki/Help:Extension:ParserFunctions#.23time here] (Arabic, Persian, Hebrew, Thai solar, Minguo, Japanese nengo). some article's data are base on these calendars and users should convert them to Gregorian to input them at Wikidata and it is difficult.
Please Develop a gadget (maybe a JS tool) which converts these calendar to Gregorian to input them at Wikidata. I didn't propose to change the database date format I only suggested a converter gadget.