There is no difference here in using Hijri or Islamic calendar.
The epoch of the Islamic calendar isn't fixed, because we cannot surely say today, when the crescent of the new moon has been observed around the July 16, 622 C.E. Julian. Even today the start and end of the month Ramadan is defined by religous authorities. So the calendar can be offset by two days.
We don't support the offset here, however we changed the epoch from "Calendrical Calculations" to value, that .Net seems to be using.
This class is not compatible to T:System.Globalization.HijriCalendar.
T:CCFixed