Week 1 Definition Algorithm

Calculation Steps:

1. Find January 4th of the target year
2. Determine what day of the week it falls on
3. Calculate the Monday of that week (this is the start of Week 1)
4. Count weeks from this starting point

from datetime import datetime, timedelta

def get_iso_week_number(date):
    """Calculate ISO 8601 week number for a given date"""
    # Find January 4th of the same year
    jan4 = datetime(date.year, 1, 4)
    
    # Find Monday of the week containing January 4th
    week1_monday = jan4 - timedelta(days=jan4.weekday())
    
    # Calculate days since Week 1 Monday
    days_diff = (date - week1_monday).days
    
    # Calculate week number
    week_num = (days_diff // 7) + 1
    
    return week_num

# Example usage
today = datetime.now()
week = get_iso_week_number(today)
print(f"Today is Week {week}")

function getISOWeekNumber(date) {
    // Clone date to avoid mutation
    const d = new Date(date);
    
    // Set to Thursday of the same week (ISO week)
    d.setDate(d.getDate() + 4 - (d.getDay() || 7));
    
    // Get first day of year
    const yearStart = new Date(d.getFullYear(), 0, 1);
    
    // Calculate week number
    const weekNum = Math.ceil(((d - yearStart) / 86400000 + 1) / 7);
    
    return weekNum;
}

// Example usage
const today = new Date();
const week = getISOWeekNumber(today);
console.log(`Today is Week ${week}`);