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 unsuited for computer calculation.
iman Pro implements an arithmetic Islamic calendar, which is an approximation of the actual calendar.
Thus, it won’t be as accurate as the calendar declared by local authorities which will vary from local to local.
However, the app does provide a tool to do a manual correction to the calendar. You can find that in the app settings and apply +1 day or -1 day to the current date.