It is a substantial undertaking, requiring a great deal of expertise, to implement the Islamic calendar. The difficulty is that there are multiple versions of the calendar. In some places the calendar is based on human observation of the new moon, and so a database of past observations is needed (and future dates can't be represented). In other places the time of observability of the new moon is calculated according to an astronomical ephemeris (and different ephemerides are used in different places and at different times).