What is the difference between NSIslamicCalendar and NSIslamicCivilCalendar?
The civil Hajiri calendar is predictable. You can know, today, how many days will elapse before the first day of the next month. This is not true for the religious calendar. It is based on being able to observe the new moon. No visible new moon means the new month has not started. Computers and coders cannot deal with that very well.
Since you are coding, if you need future dates, you will have to use the civil calendar.