by Richard Russell, August 2015

The code below shows how to convert between Western (Gregorian) and Islamic (Hijri) dates; it requires Windows Vista or later:

        year% = 2015 : month% = 8 : day% = 14
 
        PROC_GregorianToHijri(year%, month%, day%)
        PRINT year%, month%, day%
 
        PROC_HijriToGregorian(year%, month%, day%)
        PRINT year%, month%, day%
 
        END
 
        DEF PROC_GregorianToHijri(RETURN year%, RETURN month%, RETURN day%)
        LOCAL cdt{}, st{} : CAL_HIJRI = 6
        DIM st{wYear{l&,h&}, wMonth{l&,h&}, wDayOfWeek{l&,h&}, wDay{l&,h&}, \
        \   wHour{l&,h&}, wMinute{l&,h&}, wSecond{l&,h&}, wMilliseconds{l&,h&}}
        DIM cdt{CalId%, Era%, Year%, Month%, Day%, DayOfWeek%, Hour%, Minute%, \
        \   Second%, Tick%}
        st.wYear.l& = year% : st.wYear.h& = year% >> 8
        st.wMonth.l& = month% : st.wDay.l& = day%
        SYS "ConvertSystemTimeToCalDateTime", st{}, CAL_HIJRI, cdt{}
        year% = cdt.Year% : month% = cdt.Month% : day% = cdt.Day%
        ENDPROC
 
        DEF PROC_HijriToGregorian(RETURN year%, RETURN month%, RETURN day%)
        LOCAL cdt{}, st{} : CAL_HIJRI = 6
        DIM st{wYear{l&,h&}, wMonth{l&,h&}, wDayOfWeek{l&,h&}, wDay{l&,h&}, \
        \   wHour{l&,h&}, wMinute{l&,h&}, wSecond{l&,h&}, wMilliseconds{l&,h&}}
        DIM cdt{CalId%, Era%, Year%, Month%, Day%, DayOfWeek%, Hour%, Minute%, \
        \   Second%, Tick%}
        cdt.Year% = year% : cdt.Month% = month% : cdt.Day% = day%
        cdt.CalId% = CAL_HIJRI : cdt.Era% = 1
        FOR cdt.DayOfWeek% = 1 TO 7
          SYS "ConvertCalDateTimeToSystemTime", cdt{}, st{}
        NEXT
        year% = st.wYear.l& + 256*st.wYear.h&
        month% = st.wMonth.l& : day% = st.wDay.l&
        ENDPROC

Note that, depending on the time of day and the location, there may be some ambiguity in the conversion so don't be surprised if these routines give a result which is a day or so different from that obtained from other calculators.