# FC_GetWeekNumber

## Description
Calculates the **ISO 8601 week number** (week 1 = week containing first Thursday)  
Input: DT (DATE_AND_TIME)  
Output: UINT (1..53)

## Features
- Follows ISO 8601 rules (Monday week start)
- Uses built-in Siemens functions (WEEKDAY, DAY_OF_YEAR)

## Input / Output
**Input**
- `stDateTime` : DT     Date/time value

**Return**
- UINT                  Week number 1..53

## Usage example
```iecst
wCurrentWeek := "FC_GetWeekNumber"(stDateTime := #Now);
IF wCurrentWeek = 1 THEN
    // New year logic...
END_IF;