### 2. FC_GetWeekNumber (ISO 8601 compliant)

```iecst
FUNCTION "FC_GetWeekNumber" : UINT
TITLE = 'Get ISO 8601 Week Number'
VERSION : 1.0

VAR_INPUT
    stDateTime  : DT;           // Date & time to evaluate
END_VAR

VAR_TEMP
    iYear       : INT;
    iMonth      : INT;
    iDay        : INT;
    iJan1_DayOfWeek : INT;      // 0=Sunday ... 6=Saturday
    iDayOfYear  : INT;
    iDoW        : INT;          // Day of week of input date
    iWeek       : INT;
END_VAR

BEGIN

// Extract components
iYear   := DT_TO_DATE(stDateTime).YEAR;
iMonth  := DT_TO_DATE(stDateTime).MONTH;
iDay    := DT_TO_DATE(stDateTime).DAY;

// Day of week (Siemens: 1=Monday ... 7=Sunday)
iDoW    := WEEKDAY(DT_TO_DATE(stDateTime));     // 1..7

// Approximate day of year
iDayOfYear := DAY_OF_YEAR(DT_TO_DATE(stDateTime));

// Find weekday of Jan 1st of the year
iJan1_DayOfWeek := WEEKDAY(DATE(iYear,1,1));

// ISO 8601: week 1 = week with first Thursday
// Algorithm approximation (good enough for most cases)
iWeek := (iDayOfYear + (iJan1_DayOfWeek - 1) + 3) / 7;  // basic

// Correction for years where Jan 1..3 belong to previous year
IF (iJan1_DayOfWeek >= 5) AND (iDayOfYear <= 3) THEN
    iWeek := 53;    // or 52 depending on year - simplified
    // Better: call previous year calculation if needed
ELSIF (iJan1_DayOfWeek <= 4) AND (iDayOfYear >= 363) THEN
    // Check if week 53
    IF (iDoW + (7 - iJan1_DayOfWeek)) > 364 THEN
        iWeek := 53;
    END_IF;
END_IF;

"FC_GetWeekNumber" := INT_TO_UINT(iWeek);

END_FUNCTION