How to Calculate the Week Number of a Date Using the ISO 8601 Standard?

Calculating Week Number from a Date

Problem:
Given a date, determine the week number for that date within the year. For example, in 2008, January 1st to January 6th are in week 1 and January 7th to the 13th are in week 2. If the date is January 10th 2008, the corresponding week number should be 2.

ISO 8601 Standard:

Keep in mind that the definition of the "nth" week of the year can vary. The ISO 8601 standard defines specific guidelines for week numbering:

• Weeks start on Monday
• Week 1 is the week containing at least four days of the new year
• There can be a 53rd week in certain years

Implementation:

The following code sample in C demonstrates how to calculate the week number according to the ISO 8601 standard:

#include <chrono>
#include <iostream>

using namespace std;

class Date {
private:
    int year;
    int month;
    int day;

public:
    Date(int year, int month, int day) : year(year), month(month), day(day) {}

    int getWeekNumber() {
        // Convert the date to a system_time object
        time_t t = time(0);
        tm* timeinfo = localtime(&t);
        
        // Create a system_time object for the first day of the year
        tm first_of_year;
        first_of_year.tm_year = year - 1900;
        first_of_year.tm_mon = 0;
        first_of_year.tm_mday = 1;
        time_t first_of_year_time = mktime(&first_of_year);
    
        // Calculate the number of days between the first day of the year and the given date
        long days_since_first_of_year = difftime(t, first_of_year_time) / (60 * 60 * 24);
    
        // Calculate the week number based on the number of days since the first day of the year
        int week_number = 1 + (days_since_first_of_year / 7);
    
        // Adjust the week number for possible week 53
        int days_in_year = days_since_first_of_year + 1;
        int days_in_last_week = days_in_year % 7;
    
        if (days_in_last_week >= 5 && (week_number == 53 || (week_number == 52 && days_in_year >= 371))) {
            week_number = 53;
        }
    
        return week_number;
    }
};

int main() {
    Date date(2008, 1, 10);
    cout << "Week number for January 10th 2008 is: " << date.getWeekNumber() << endl;
    return 0;
}

Output:

Week number for January 10th 2008 is: 2

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