This code didn't give me the week number according to ISO8601 so I changed it a bit.

I added some code to define January first of the current year and change the week number according to ISO 8601 (first thursday is always week 1)

ThisJear1Jan =>  DTL
Temp1Jan       =>  DTL
DateString       =>  String
Error                 =>  Bool
status               =>  Word
Separator        =>  Char

#TMP_RD := RD_SYS_T(#TMP_DTL);

#Temp1Jan:=#TMP_DTL;
#"Temp1Jan".NANOSECOND:=00;
#"Temp1Jan".SECOND:=00;
#"Temp1Jan".MINUTE:=00;
#"Temp1Jan".HOUR:=00;
#"Temp1Jan".DAY:=01;
#"Temp1Jan".MONTH:=01;

// This conversion is done because simply changing the jear in the DTL struct doesn't change the weekday
#Separator:='-';
#DateString:="LGF_DTLtoString_ISO"("date":=#Temp1Jan, separator:=#Separator);
#ThisJear1Jan:="LGF_StringToDTL_ISO"("date":=#DateString, error=>#Error, status=>#status);

IF #TMP_DTL.MONTH <> 1 THEN // If January has been passed (at least one complete month has elapsed)
#month_31 := #TMP_DTL.MONTH / 2; // Calculate the number of 31 day months passed (assuming alternating 31, 30 day month pattern)
IF (#TMP_DTL.MONTH MOD 2) = 0 THEN // If even month (assuming odd months = 31 days and even months = 30 days)
#month_30 := #month_31 - 1; // Number of 30 day months passed is one less than the number of 31 days months passed
ELSE // Otherwise (odd month)
#month_30 := #month_31; // Number of 30 day months passed equals the number of 31 days months passed
END_IF;
#Elapsed_Days := (#month_31*31) + (#month_30*30); // Calculate number of elapsed days in complete elapsed months
IF #TMP_DTL.MONTH <> 2 THEN // If the complete month of February has been passed
IF (#TMP_DTL."YEAR" MOD 4) = 0 THEN // If this year is a leap year...
#Elapsed_Days := #Elapsed_Days - 1; // ...then February has 29 days, so reduce already added 30 day month by 1
ELSE // ...Otherwise, it is not a leap year
#Elapsed_Days := #Elapsed_Days - 2; // ...so February has 28 days, so reduce already added 30 day month by 2
END_IF;
END_IF;
IF #TMP_DTL.MONTH = 9 OR #TMP_DTL.MONTH = 11 THEN // If month is September or November
#Elapsed_Days := #Elapsed_Days + 1; // Take into account the July/August special case of non alternating 30, 31 month pattern
END_IF;
END_IF;

#OrdinalDay := #Elapsed_Days + #TMP_DTL.DAY; // Add in the number of elapsed days in the current month
IF #ThisJear1Jan.WEEKDAY>=5 THEN #Week_num := ((#OrdinalDay+#ThisJear1Jan.WEEKDAY-3) / 7);
ELSE #Week_num := ((#OrdinalDay+#ThisJear1Jan.WEEKDAY+4) / 7);
END_IF;