The user has set their preferences to use Jalali (aka Persian) calendar and this correctly reflects in Watchlist, RecentChanges and page history. But notifications from Echo are still showing him dates in Gregorian (default) calendar.
My understanding is that the calendar is not really part of Notifications but is powered by a more fundamental layer of the system. The thinking of the team was that if WMF wants to allow for alternative calendars, then that goes beyond Collaboration Team's scope of authority and responsibility. @Catrope could explain better. I see this is already part of "MediaWiki Internationalization" and "Mediawiki User preferences." I don't know what team has responsibility for these project, but maybe this ball is properly in their court?