I've almost completed my QA review of the code in the existing Calendar Systems, with the Hijri / Islamic calendar passing relatively unscathed but some possible optimisations noted (it uses recursion where direct calculation may be faster). The Jalali calendar (aka Iranian or Persian, as used in Iran and Afganistan) has proven somewhat harder and needed a lot of research.
The basic problem is that the Jalali is a purely Solar calendar with the first day of the year being officially defined in relation to the vernal equinox and solar noon at the reference longitude for Iran Standard Time (52°30' E). There is no offically defined rule for calculating leap years, nor can there be, a leap year is just any year in which there are 366 days between successive vernal equinoxes. The upshot is there is no simple arithmetic method to calculate when leap years should occur, for complete accuracy you must correctly calculate the vernal equinox.
There are two popular approximations for sequences of leap years, the 33 year cycle and the 2820 year cycle (aka the Birashk method), but these each return wrong results in various and different years, and there is much debate on which is better. Most implementations choose to use one of these for efficiency and simplicity regardless of accuracy, Microsoft has chosen the 33 year cycle which is more accurate in years immediately around AD 2000, many FOSS projects have gone with the 2820 year cycle due to Emacs implementing it first. I haven't confirmed which Apple supports.
In KDE we have been using the 2820 year cycle when calculating date conversions, but were using the 33 year cycle when calculating leap years, leading to some quirky errors. I've fixed that to always use the 2830 year cycle, and now restrict the valid date range to the years AP 1244 to 1530 (roughly AD 1865 to 2152), a useful range of modern dates during which the 2830 year cycle is only wrong twice (AP 1404 and 1437, roughly 2004 and 2058). I've added special exceptions to cover these two cases so we now exactly match the true astronomical calendar for the entire supported period.
In 4.5 we should begin to support astronomical calendars properly, so we will switch Jalali to correctly calculating the equinox and thus extend the supported date range. We will also provide reference implementations of the 33 and 2820 year cycle calendars to allow for interoperation with systems that use them.
The final calendar for QA is the Hebrew calendar, which looks to have had some bad bugs creep in.