@*ECHO OFF
*SETLOCAL
*UNALIAS *

:: ISOWeekLng.btm V-1.20 May 2005 - Michael Bednarek
:: Computes ISO 8601 Week Number (long and with comments)
:: Input: A legal date (>=1980-01-01, <=2099-12-31); no check.
:: Output: yyyy*100 + ww

:: Convert 4NT's DoWI (Sun=1, ... Sat=7) to ISO (Mon=1 ... Sun=7)
FUNCTION ISODOWI=`%@EVAL[(%@DOWI[%1] + 5) MOD 7 + 1]`

:: The argument's closest Thursday
SET ThuNow=%@MAKEDATE[%@EVAL[%@DATE[%1] + 4 - %@ISODOWI[%1]],4]
SET theYear=%@YEAR[%ThuNow]

:: The 1st Thursday of the year of the argument's closest Thursday
SET Thu1st=%@MAKEDATE[%@EVAL[%@DATE[%theYear-01-04] + 4 - %@ISODOWI[%theYear-01-04]],4]

:: Difference between those two Thursdays
SET ThuDif=%@EVAL[%@DATE[%ThuNow] - %@DATE[%Thu1st]]
SET theWeek=%@EVAL[%ThuDif \ 7 +1]

SET ISOWeek=%@EVAL[%theYear * 100 + %theWeek]
IF %_BATCH LT 2 ECHO ISO Week: %ISOWeek
QUIT %ISOWeek