BBC BASIC Programmers' Reference

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