As I see in the wiki opening hours should be in Gregorian calendar only. But in fact there are many countries and religions using another calendars that affect the opening hours.
We (Muslims) used the Hijri Calendar https://en.wikipedia.org/wiki/Islamic_calendar (lunar calendar) for our holidays (Eid Alfiter and Eid Aledha) all government offices and shops has another opening time in these holidays. Also in Ramdan holy month (9th month) all government offices and shops has a different opening time. So opening_hours syntax must accept the other calendar systems. the renderers may use ICU open source library to manage these other calendars see http://userguide.icu-project.org/datetime/calendar الحصول على Outlook for Android<https://aka.ms/ghei36>
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