How to Get the Correct ISO-8601 Week Number for Any Given Date?

Get the correct week number for a given date according to ISO-8601 standard

Challenge:

Determining the correct number of weeks for a given date can be tricky, especially when dealing with end-of-year dates. Existing .NET methods often fail to comply with the ISO-8601 standard and assign incorrect week numbers to some dates.

Wrong week number:

For example, the .NET approach might mark December 31, 2012 as week 53, but this is incorrect according to the ISO-8601 standard. This difference stems from the fact that .NET allows weeks to span years, while ISO-8601 does not.

Solution:

To get the correct ISO-8601 week number, use the following function:

<code class="language-csharp">// 假设周从星期一开始。
// 第 1 周是当年包含星期四的第一周。
public static int GetIso8601WeekOfYear(DateTime time)
{
    DayOfWeek day = CultureInfo.InvariantCulture.Calendar.GetDayOfWeek(time);
    if (day >= DayOfWeek.Monday && day <= DayOfWeek.Wednesday)
    {
        time = time.AddDays(3 - (int)day);
    }
    else if (day > DayOfWeek.Wednesday)
    {
        time = time.AddDays(10 - (int)day);
    }
    return CultureInfo.InvariantCulture.Calendar.GetWeekOfYear(time, CalendarWeekRule.FirstFourDayWeek, DayOfWeek.Monday);
}</code>

Principle description:

This function adjusts the date when it falls on a Monday, Tuesday, or Wednesday, ensuring that it represents the same week as the upcoming Thursday, Friday, or Saturday. This method complies with the ISO-8601 standard, which defines week 1 as the first week that contains a Thursday.

Example:

For the December 31, 2012 date mentioned in the challenge, this function returns 1, which is the correct week number according to the ISO-8601 standard.

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