How to Accurately Determine the Week Number of a Date?

Precisely Calculating the Week Number of a Date

Calculating a date's week number is a frequent programming task, but surprisingly complex. Consider 2012-12-31: many methods incorrectly return 53, while intuitively, it should be week 1.

The Problem: Calendar Variations

The core issue lies in calendar inconsistencies. Standard .NET calendars allow weeks to cross year boundaries, unlike the ISO 8601 standard, which keeps weeks within a single year.

The ISO 8601 Solution

To ensure accuracy, using the ISO 8601 standard is essential. The following GetIso8601WeekOfYear method, compliant with this standard, correctly identifies 2012-12-31 as week 1.

<code class="language-csharp">public static int GetIso8601WeekOfYear(DateTime time)
{
    DayOfWeek day = CultureInfo.InvariantCulture.Calendar.GetDayOfWeek(time);
    if (day >= DayOfWeek.Monday && day <= DayOfWeek.Wednesday)
    {
        time = time.AddDays(3);
    }
    return CultureInfo.InvariantCulture.Calendar.GetWeekOfYear(time, CalendarWeekRule.FirstFourDayWeek, DayOfWeek.Monday);
}</code>

This method guarantees week number calculation according to ISO 8601, offering consistent and logical results.

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