by Richard Russell, November 2015

The code listed below finds the ISO week number (or year and week) corresponding to a given date:

        INSTALL @lib$+"DATELIB"
 
        PRINT FN_ISOweek(1,1,2005)
        PRINT FN_ISOweek(2,1,2005)
        PRINT FN_ISOweek(31,12,2005)
        PRINT FN_ISOweek(31,12,2006)
        PRINT FN_ISOweek(1,1,2007)
        PRINT FN_ISOweek(30,12,2007)
        PRINT FN_ISOweek(31,12,2007)
        PRINT FN_ISOweek(1,1,2008)
        PRINT FN_ISOweek(28,12,2008)
        PRINT FN_ISOweek(29,12,2008)
        PRINT FN_ISOweek(30,12,2008)
        PRINT FN_ISOweek(31,12,2008)
        PRINT FN_ISOweek(1,1,2009)
        PRINT FN_ISOweek(31,12,2009)
        PRINT FN_ISOweek(1,1,2010)
        PRINT FN_ISOweek(2,1,2010)
        PRINT FN_ISOweek(3,1,2010)
 
        REM FOR d% = 1 TO 30
        REM PRINT d%, FN_ISOweek(d%,11,2015)
        REM NEXT
        END
 
        DEF FN_ISOweek(day%,month%,year%)
        LOCAL iyr%, mjd%
        iyr% = year%
        mjd% = FN_mjd(day%,month%,iyr%)
        IF mjd% >= FN_ISOwk1(iyr%+1) THEN iyr% += 1
        IF mjd% < FN_ISOwk1(iyr%) THEN iyr% -= 1
        = (mjd% - FN_ISOwk1(iyr%)) DIV 7 + 1
 
        DEF FN_ISOwk1(year%)
        LOCAL dow%, mjd%
        mjd% = FN_mjd(4,1,year%)
        dow% = FN_dow(mjd%)
        IF dow% = 0 dow% = 7
        = mjd% - dow% + 1

If you want to know the ISO year as well as the week, you can return it in an additional parameter:

        DEF FN_ISOweek(day%,month%,year%,RETURN iyr%)
        LOCAL mjd%
        ... remainder as above