What is the method for converting dates from any of the 5 Islamic (Hijri) Calendars to any of the 18 World Calendars without using External Libraries or complex astronomical formulas?

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One way to convert dates from any of the 5 Islamic (Hijri) Calendars to any of the 18 World Calendars without using External Libraries or complex astronomical formulas is to create lookup tables or algorithms for each calendar conversion.

For example, to convert a date from the Islamic lunar calendar to the Gregorian calendar, you would use a lookup table that maps each lunar year to a corresponding range of Gregorian years. Then, within that range, you would use another lookup table to map each lunar month to a corresponding Gregorian month. Finally, you would use a formula to calculate the number of days between the first day of the lunar year and the desired date within that year, and add that number of days to the corresponding Gregorian date.

Similar lookup tables and algorithms could be created for each conversion between the Islamic calendars and the other world calendars. However, it should be noted that these conversions may not be exact due to variations in the length of a year and a day between different calendars, and may need to be adjusted periodically to account for these variations.