An iOS app that I am working upon has some date based functionalities. It is working when the Calendar is set as Gregorian. But in the device if I change the calendar to anything else (i.e. Japanese or Buddhist), it doesn't work. Probably because I haven't coded to account for it.
But what is the general standard. Is it expected that any date and time based app support all calendar formats, do a large no. of such apps support all calendars or would an app be approved if it does not work with other calendar types. And if it is approved, where should it be specified that the app works only with a certain calendar type so that the user knows about it?
There is no official requirement from Apple that all calendars should be supported. Use common sense and do what makes sense for your target audience. If your target audience is Western countries (or the entire world except a small population of Japanese/Buddhist people), then just support Gregorian. If your app is designed specifically for Japanese or Far East regions, support those calendars. Do what applies to the 90% case, and don't waste your time optimizing for marginal use cases. I have shipped many apps that leveraged Calendar and NSDate functionality, and I've never supported anything but Gregorian. I've never heard a complaint.