C# 根据周数计算日期

任何人都知道一个简单的方法来获取一周中第一天的日期（欧洲的星期一）。我知道年份和周数吗？我将在C#中这样做。

最简单的方法可能是找到一年中的第一个星期一，然后添加相关的周数。下面是一些示例代码。顺便说一句，它假设一个从1开始的星期数：

using System;

class Test
{
    static void Main()
    {
        // Show the third Tuesday in 2009. Should be January 20th
        Console.WriteLine(YearWeekDayToDateTime(2009, DayOfWeek.Tuesday, 3));
    }

    static DateTime YearWeekDayToDateTime(int year, DayOfWeek day, int week)
    {
        DateTime startOfYear = new DateTime (year, 1, 1);

        // The +7 and %7 stuff is to avoid negative numbers etc.
        int daysToFirstCorrectDay = (((int)day - (int)startOfYear.DayOfWeek) + 7) % 7;

        return startOfYear.AddDays(7 * (week-1) + daysToFirstCorrectDay);
    }
}

---

第一周定义为从周一开始的一周，包含一年中的第一个周四。

注意 下面的答案使用.NET日历规则。它不保证符合ISO8601标准。当你需要的时候，看看这里的其他答案。周数是一团乱，一定要先找出你需要遵循的规则

---

下面的代码正确地将2009年第1周的开始时间设置为2008年12月29日。CalendarWeekRule可能应该是一个参数

请注意，weekNum应大于等于1

static DateTime FirstDateOfWeek(int year, int weekNum, CalendarWeekRule rule)
{
    Debug.Assert(weekNum >= 1);

    DateTime jan1 = new DateTime(year, 1, 1);

    int daysOffset = DayOfWeek.Monday - jan1.DayOfWeek;
    DateTime firstMonday = jan1.AddDays(daysOffset);
    Debug.Assert(firstMonday.DayOfWeek == DayOfWeek.Monday);

    var cal = CultureInfo.CurrentCulture.Calendar;
    int firstWeek = cal.GetWeekOfYear(firstMonday, rule, DayOfWeek.Monday);

    if (firstWeek <= 1)
    {
        weekNum -= 1;
    }

    DateTime result = firstMonday.AddDays(weekNum * 7);

    return result;
}

静态日期时间FirstDateOfWeek（整数年、整数周、日历周规则）
{
Assert（weekNum>=1）；
DateTime jan1=新的日期时间（年，1，1）；
int daysofset=DayOfWeek.Monday-jan1.DayOfWeek；
DateTime firstMonday=1月1日。添加天数（Daysofset）；
Assert（firstMonday.DayOfWeek==DayOfWeek.Monday）；
var cal=CultureInfo.CurrentCulture.Calendar；
int firstWeek=cal.GetWeekOfYear（第一个星期一，规则，星期一）；
如果（第一周个人而言，我会利用文化信息获取一周中的某一天，并循环到该文化在一周中的第一天。我不确定我是否正确解释了它，下面是一个示例：

    public DateTime GetFirstDayOfWeek(int year, int weekNumber)
    {
        return GetFirstDayOfWeek(year, weekNumber, Application.CurrentCulture);
    }

    public DateTime GetFirstDayOfWeek(int year, int weekNumber,
        System.Globalization.CultureInfo culture)
    {
        System.Globalization.Calendar calendar = culture.Calendar;
        DateTime firstOfYear = new DateTime(year, 1, 1, calendar);
        DateTime targetDay = calendar.AddWeeks(firstOfYear, weekNumber);
        DayOfWeek firstDayOfWeek = culture.DateTimeFormat.FirstDayOfWeek;

        while (targetDay.DayOfWeek != firstDayOfWeek)
        {
            targetDay = targetDay.AddDays(-1);
        }

        return targetDay;
    }

根据瑞典使用的ISO 8601:1988，一年中的第一周是新年中至少有四天的第一周

因此，如果你的一周从周一开始，那么一年中的第一个周四就在第一周之内。
您可以从中选择DateAdd或DateDiff。
我喜欢Henk Holterman提供的解决方案。但为了更独立于区域性，您必须获得当前区域性的每周第一天（不总是周一）：

使用系统全球化；
静态日期时间FirstDateOfWeek（整年、整周）
{
DateTime jan1=新的日期时间（年，1，1）；
int daysofset=（int）CultureInfo.CurrentCulture.DateTimeFormat.FirstDayOfWeek-（int）jan1.DayOfWeek；
DateTime firstMonday=1月1日。添加天数（Daysofset）；
int firstWeek=CultureInfo.CurrentCulture.Calendar.GetWeekOfYear（1月1日，CultureInfo.CurrentCulture.DateTimeFormat.CalendarWeekRule，CultureInfo.CurrentCulture.DateTimeFormat.FirstDayOfWeek）；
如果（第一周使用流畅的日期时间

要在两个方向上进行转换，请参见此处：
假设周数从1开始

DateTime dt =  new DateTime(YearNumber, 1, 1).AddDays((WeekNumber - 1) * 7 - (WeekNumber == 1 ? 0 : 1));
return dt.AddDays(-(int)dt.DayOfWeek);

这将为您提供一周中的第一天。我没有对它做过很多测试，但看起来它很有效。它比我在web上找到的大多数其他解决方案都小，因此希望与大家分享。
我对Thomas的解决方案做了一些改进，添加了一个覆盖：

   public static DateTime FirstDateOfWeek(int year, int weekOfYear)
    {
      return Timer.FirstDateOfWeekOfMonth(year, 1, weekOfYear);
    }

    public static DateTime FirstDateOfWeekOfMonth(int year, int month, 
    int weekOfYear)
    {
      DateTime dtFirstDayOfMonth = new DateTime(year, month, 1);

       //I also commented out this part:
      /*
      if (firstWeek <= 1)
      {
        weekOfYear -= 1;
      }
      */

publicstaticdatetime FirstDateOfWeek（int-year，int-weekOfYear）
{
返回计时器。FirstDateOfWeekOfMonth（year，1，weekOfYear）；
}
公共静态日期时间FirstDateOfWeekOfMonth（整数年、整数月、，
国际周（年）
{
DateTime dtFirstDayOfMonth=新的日期时间（年、月、1）；
//我也评论了这一部分：
/*
如果（第一周免费课程包括合规课程周：

我尝试了上面的一些代码，有些代码有小错误，当你尝试不同的年份和不同的开始日期时，你会看到它们，我用了Jon Skeet的代码，修复了它，它工作了，非常简单的代码

Public Function YearWeekDayToDateTime(ByVal year As Integer, ByVal weekDay As Integer, ByVal week As Integer) As DateTime
   ' weekDay, day you want
    Dim startOfYear As New DateTime(year, 1, 1)
    Dim startOfYearFixDay As Integer

    If startOfYear.DayOfWeek <> DayOfWeek.Sunday Then
        startOfYearFixDay = startOfYear.DayOfWeek
    Else
        startOfYearFixDay = 7
    End If

    Return startOfYear.AddDays((7 * (week)) - startOfYearFixDay + weekDay)
End Function

公共函数yearkdaytodatetime（ByVal year作为整数，ByVal weekDay作为整数，ByVal week作为整数）作为日期时间
“工作日，你想要的日子
Dim startOfYear作为新日期时间（年，1，1）
Dim startOfYearFixDay作为整数
如果开始是一年。星期天。星期天。那么
startOfYearFixDay=startOfYear.DayOfWeek
其他的
startOfYearFixDay=7
如果结束
返回startOfYear.AddDays（（7*（周））-startOfYearFixDay+工作日）
端函数
我对@Henkholtman的解决方案有意见，甚至对@RobinAndersson的修复也有意见

阅读ISO 8601标准很好地解决了这个问题。将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年第53周

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);
}       

我使用了其中一种解决方案，但它给了我错误的结果，原因很简单，因为它将星期天计算为一周的第一天

int Year = 2014;
int Week = 48;
int DayOfWeek = 4;

DateTime FecIni = new DateTime(Year, 1, 1);
FecIni = FecIni.AddDays(7 * (Week - 1));
if ((int)FecIni.DayOfWeek > DayOfWeek)
{
    while ((int)FecIni.DayOfWeek != DayOfWeek) FecIni = FecIni.AddDays(-1);
}
else
{
    while ((int)FecIni.DayOfWeek != DayOfWeek) FecIni = FecIni.AddDays(1);
}

我改变了：

var firstDay = new DateTime(DateTime.Now.Year, 1, 1).AddDays((weekNumber - 1) * 7);
var lastDay = firstDay.AddDays(6);

致：

现在它正在发挥作用。
建议的解决方案不完整-它仅适用于CalendarWeekRule.FirstFullWeek。其他类型的周规则不起作用。可以使用此测试用例看到这一点：

foreach (CalendarWeekRule rule in Enum.GetValues(typeof(CalendarWeekRule)))
{
    for (int year = 1900; year < 2000; year++)
    {
        DateTime date = FirstDateOfWeek(year, 1, rule);
        Assert(CultureInfo.CurrentCulture.Calendar.GetWeekOfYear(date, rule, DayOfWeek.Monday) == 1);
        Assert(CultureInfo.CurrentCulture.Calendar.GetWeekOfYear(date.AddDays(-1), rule, DayOfWeek.Monday) != 1);
    }
}

foreach（Enum.GetValues中的CalendarWeekRule规则（typeof（CalendarWeekRule）））
{
对于（整数年=1900；年份<2000；年份++）
{
DateTime日期=第一个星期的日期（第1年，规则）；
断言（CultureInfo.CurrentCulture.Calendar.GetWeekOfYear（日期、规则、星期一的日期）=1）；
断言（CultureInfo.CurrentCulture.Calendar.GetWeekOfYear（date.AddDays（-1），rule，DayOfWeek.Monday）！=1）；
}
}
我已经为提议的解决方案制作了一个改进版本，该版本更简单，并将第一天的工作参数化：

public static DateTime GetFirstDayOfWeek(int year, int week, DayOfWeek firstDayOfWeek)
{
    return GetWeek1Day1(year, firstDayOfWeek).AddDays(7 * (week - 1));
}

public static DateTime GetWeek1Day1(int year, DayOfWeek firstDayOfWeek)
{
    DateTime date = new DateTime(year, 1, 1);

    // Move towards firstDayOfWeek
    date = date.AddDays(firstDayOfWeek - date.DayOfWeek);

    // Either 1 or 52 or 53
    int weekOfYear = CultureInfo.CurrentCulture.Calendar.GetWeekOfYear(date, CalendarWeekRule.FirstFullWeek, firstDayOfWeek);

    // Move forwards 1 week if week is 52 or 53
    date = date.AddDays(7 * System.Math.Sign(weekOfYear - 1));

    return date;
}

这一个对我有用，它还有一个优点，就是希望cultureinfo作为参数来测试具有不同区域性的公式。如果为空，它将获取当前区域性信息…有效值如：“it”、“en-us”、“fr”…ando等等。
诀窍是减去一年中第一天的周数，可能是1，表示第一天在第一周内。希望这有帮助

Public Shared Function FirstDayOfWeek(ByVal year As Integer, ByVal weekNumber As Integer, ByVal culture As String) As Date
    Dim cInfo As System.Globalization.CultureInfo
    If culture = "" Then
        cInfo = System.Globalization.CultureInfo.CurrentCulture
    Else
        cInfo = System.Globalization.CultureInfo.CreateSpecificCulture(culture)
    End If
    Dim calendar As System.Globalization.Calendar = cInfo.Calendar
    Dim firstOfYear As DateTime = New DateTime(year, 1, 1, calendar)
    Dim firstDayWeek As Integer = calendar.GetWeekOfYear(firstOfYear, cInfo.DateTimeFormat.CalendarWeekRule, cInfo.DateTimeFormat.FirstDayOfWeek)
    weekNumber -= firstDayWeek
    Dim targetDay As DateTime = calendar.AddWeeks(firstOfYear, weekNumber)
    Dim fDayOfWeek As DayOfWeek = cInfo.DateTimeFormat.FirstDayOfWeek

    While (targetDay.DayOfWeek <> fDayOfWeek)
        targetDay = targetDay.AddDays(-1)
    End While
    Return targetDay
End Function

公共共享函数FirstDayOfWeek（ByVal year作为整数，ByVal weekNumber作为整数，ByVal区域性作为字符串）作为日期
Dim cInfo As System.Globalization.CultureInfo
如果culture=”“，则
cInfo=System.Globalization.CultureInfo.CurrentCulture
其他的
foreach (CalendarWeekRule rule in Enum.GetValues(typeof(CalendarWeekRule)))
{
    for (int year = 1900; year < 2000; year++)
    {
        DateTime date = FirstDateOfWeek(year, 1, rule);
        Assert(CultureInfo.CurrentCulture.Calendar.GetWeekOfYear(date, rule, DayOfWeek.Monday) == 1);
        Assert(CultureInfo.CurrentCulture.Calendar.GetWeekOfYear(date.AddDays(-1), rule, DayOfWeek.Monday) != 1);
    }
}

public static DateTime GetFirstDayOfWeek(int year, int week, DayOfWeek firstDayOfWeek)
{
    return GetWeek1Day1(year, firstDayOfWeek).AddDays(7 * (week - 1));
}

public static DateTime GetWeek1Day1(int year, DayOfWeek firstDayOfWeek)
{
    DateTime date = new DateTime(year, 1, 1);

    // Move towards firstDayOfWeek
    date = date.AddDays(firstDayOfWeek - date.DayOfWeek);

    // Either 1 or 52 or 53
    int weekOfYear = CultureInfo.CurrentCulture.Calendar.GetWeekOfYear(date, CalendarWeekRule.FirstFullWeek, firstDayOfWeek);

    // Move forwards 1 week if week is 52 or 53
    date = date.AddDays(7 * System.Math.Sign(weekOfYear - 1));

    return date;
}

Public Shared Function FirstDayOfWeek(ByVal year As Integer, ByVal weekNumber As Integer, ByVal culture As String) As Date
    Dim cInfo As System.Globalization.CultureInfo
    If culture = "" Then
        cInfo = System.Globalization.CultureInfo.CurrentCulture
    Else
        cInfo = System.Globalization.CultureInfo.CreateSpecificCulture(culture)
    End If
    Dim calendar As System.Globalization.Calendar = cInfo.Calendar
    Dim firstOfYear As DateTime = New DateTime(year, 1, 1, calendar)
    Dim firstDayWeek As Integer = calendar.GetWeekOfYear(firstOfYear, cInfo.DateTimeFormat.CalendarWeekRule, cInfo.DateTimeFormat.FirstDayOfWeek)
    weekNumber -= firstDayWeek
    Dim targetDay As DateTime = calendar.AddWeeks(firstOfYear, weekNumber)
    Dim fDayOfWeek As DayOfWeek = cInfo.DateTimeFormat.FirstDayOfWeek

    While (targetDay.DayOfWeek <> fDayOfWeek)
        targetDay = targetDay.AddDays(-1)
    End While
    Return targetDay
End Function

// Google Analytics does not follow ISO standards for date.
// It numbers week 1 starting on Jan. 1, regardless what day of week it starts on.
// It treats Sunday as the first day of the week.
// The first and last weeks of a year are usually not complete weeks.
public static DateTime GetStartDateTimeFromWeekNumberInYear(int year, uint weekOfYear)
{
  if (weekOfYear == 0 || weekOfYear > 54) throw new ArgumentException("Week number must be between 1 and 54! (Yes, 54... Year 2000 had Jan. 1 on a Saturday plus 53 Sundays.)");

  // January 1 -- first week.
  DateTime firstDayInWeek = new DateTime(year, 1, 1);
  if (weekOfYear == 1) return firstDayInWeek;

  // Get second week, starting on the following Sunday.      
  do
  {
    firstDayInWeek = firstDayInWeek.AddDays(1);
  } while (firstDayInWeek.DayOfWeek != DayOfWeek.Sunday);

  if (weekOfYear == 2) return firstDayInWeek;

  // Now get the Sunday of whichever week we're looking for.
  return firstDayInWeek.AddDays((weekOfYear - 2)*7);
}

int Year = 2014;
int Week = 48;
int DayOfWeek = 4;

DateTime FecIni = new DateTime(Year, 1, 1);
FecIni = FecIni.AddDays(7 * (Week - 1));
if ((int)FecIni.DayOfWeek > DayOfWeek)
{
    while ((int)FecIni.DayOfWeek != DayOfWeek) FecIni = FecIni.AddDays(-1);
}
else
{
    while ((int)FecIni.DayOfWeek != DayOfWeek) FecIni = FecIni.AddDays(1);
}

public static DateTime FirstDateOfWeekIso8601(int year, int week)
{
        var firstThursdayOfYear = new DateTime(year, 1, 1);
        while (firstThursdayOfYear.DayOfWeek != DayOfWeek.Thursday)
        {
            firstThursdayOfYear = firstThursdayOfYear.AddDays(1);
        }

        var startDateOfWeekOne = firstThursdayOfYear.AddDays(-(DayOfWeek.Thursday - DayOfWeek.Monday));

        return startDateOfWeekOne.AddDays(7 * (week - 1));        
}

public static DateTime GetFirstDateOfWeekByWeekNumber(int year, int weekNumber)
        {
            var date = new DateTime(year, 01, 01);
            var firstDayOfYear = date.DayOfWeek;
            var result = date.AddDays(weekNumber * 7);

            if (firstDayOfYear == DayOfWeek.Monday)
                return result.Date;
            if (firstDayOfYear == DayOfWeek.Tuesday)
                return result.AddDays(-1).Date;
            if (firstDayOfYear == DayOfWeek.Wednesday)
                return result.AddDays(-2).Date;
            if (firstDayOfYear == DayOfWeek.Thursday)
                return result.AddDays(-3).Date;
            if (firstDayOfYear == DayOfWeek.Friday)
                return result.AddDays(-4).Date;
            if (firstDayOfYear == DayOfWeek.Saturday)
                return result.AddDays(-5).Date;
            return result.AddDays(-6).Date;
        }

    /// <summary>
    /// Converts a date to a week number.
    /// ISO 8601 week 1 is the week that contains the first Thursday that year.
    /// </summary>
    public static int ToIso8601Weeknumber(this DateTime date)
    {
        var thursday = date.AddDays(3 - date.DayOfWeek.DayOffset());
        return (thursday.DayOfYear - 1) / 7 + 1;
    }

    /// <summary>
    /// Converts a week number to a date.
    /// Note: Week 1 of a year may start in the previous year.
    /// ISO 8601 week 1 is the week that contains the first Thursday that year, so
    /// if December 28 is a Monday, December 31 is a Thursday,
    /// and week 1 starts January 4.
    /// If December 28 is a later day in the week, week 1 starts earlier.
    /// If December 28 is a Sunday, it is in the same week as Thursday January 1.
    /// </summary>
    public static DateTime FromIso8601Weeknumber(int weekNumber, int? year = null, DayOfWeek day = DayOfWeek.Monday)
    {
        var dec28 = new DateTime((year ?? DateTime.Today.Year) - 1, 12, 28);
        var monday = dec28.AddDays(7 * weekNumber - dec28.DayOfWeek.DayOffset());
        return monday.AddDays(day.DayOffset());
    }

    /// <summary>
    /// Iso8601 weeks start on Monday. This returns 0 for Monday.
    /// </summary>
    private static int DayOffset(this DayOfWeek weekDay)
    {
        return ((int)weekDay + 6) % 7;
    }

 DateTime GetFirstWeekDay(int year, int weekNum)
    {
        Calendar calendar = CultureInfo.CurrentCulture.Calendar;

        DateTime jan1 = new DateTime(year, 1, 1);

        int daysOffset = DayOfWeek.Monday - jan1.DayOfWeek;
        DateTime firstMonday = jan1.AddDays(daysOffset);
        int firstMondayWeekNum = calendar.GetWeekOfYear(firstMonday, CalendarWeekRule.FirstFourDayWeek, DayOfWeek.Monday);

        DateTime firstWeekDay = firstMonday.AddDays((weekNum-firstMondayWeekNum) * 7);

        return firstWeekDay;
    }