The Islamic calendar system is a lunar calendar based on observation. The observation aspect of the calendar means that a new month can only be declared based on human observations of the moon, something which can obviously vary and is unsuited to computer calculation.
The app implements arithmetic Islamic calendar, which is an approximation of the actual calendar. Thus, it wont be as accurate as the calendar declared by local authorities vary from place to place.
However, the app does provide a tool to do manual correction to the calendar. You can find that in the app settings:
iman Settings > Hijri Date Adjustements > Add 1 day