从周数计算日期 [英] Calculate date from week number

问题描述

任何人都知道一种简单的方法来获取一周中的第一天(欧洲的星期一)的日期.我知道年份和星期几?我将在 C# 中执行此操作.

Anyone know an easy way to get the date of the first day in the week (monday here in Europe). I know the year and the week number? I'm going to do this in C#.

推荐答案

我对@HenkHolterman 的解决方案有问题，即使@RobinAndersson 进行了修复.

I had issues with the solution by @HenkHolterman even with the fix by @RobinAndersson.

阅读 ISO 8601 标准可以很好地解决这个问题.使用第一个星期四作为目标，而不是星期一.下面的代码也适用于 2009 年的第 53 周.

Reading up on the ISO 8601 standard resolves the issue nicely. Use the first Thursday as the target and not Monday. The code below will work for Week 53 of 2009 as well.

public static DateTime FirstDateOfWeekISO8601(int year, int weekOfYear)
{
    DateTime jan1 = new DateTime(year, 1, 1);
    int daysOffset = DayOfWeek.Thursday - jan1.DayOfWeek;

    // Use first Thursday in January to get first week of the year as
    // it will never be in Week 52/53
    DateTime firstThursday = jan1.AddDays(daysOffset);
    var cal = CultureInfo.CurrentCulture.Calendar;
    int firstWeek = cal.GetWeekOfYear(firstThursday, CalendarWeekRule.FirstFourDayWeek, DayOfWeek.Monday);

    var weekNum = weekOfYear;
    // As we're adding days to a date in Week 1,
    // we need to subtract 1 in order to get the right date for week #1
    if (firstWeek == 1)
    {
        weekNum -= 1;
    }

    // Using the first Thursday as starting week ensures that we are starting in the right year
    // then we add number of weeks multiplied with days
    var result = firstThursday.AddDays(weekNum * 7);

    // Subtract 3 days from Thursday to get Monday, which is the first weekday in ISO8601
    return result.AddDays(-3);
}       

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