使用DatePart()的ISO8601周数不正确

Question

我班上有以下Sub.我没有错误,但结果与ISO-8601标准不符

Private Sub calculateAllProperties(ByVal dt As Date)

    Select Case dt.DayOfWeek

        Case DayOfWeek.Monday

            m_CurrentWeekStartDate = dt

        Case DayOfWeek.Tuesday

            m_CurrentWeekStartDate = DateAdd(DateInterval.Day, -1, dt)

        Case DayOfWeek.Wednesday

            m_CurrentWeekStartDate = DateAdd(DateInterval.Day, -2, dt)

        Case DayOfWeek.Thursday

            m_CurrentWeekStartDate = DateAdd(DateInterval.Day, -3, dt)

        Case DayOfWeek.Friday

            m_CurrentWeekStartDate = DateAdd(DateInterval.Day, -4, dt)

        Case DayOfWeek.Saturday

            m_CurrentWeekStartDate = DateAdd(DateInterval.Day, -5, dt)

        Case DayOfWeek.Sunday

            m_CurrentWeekStartDate = DateAdd(DateInterval.Day, -6, dt)

    End Select

    'Now we have our start point of m_CurrentWeekStartDate we can calculate all other properties.

    m_CurrentWeekStartYear = DatePart(DateInterval.Year, m_CurrentWeekStartDate)

    m_CurrentWeekNo = DatePart(DateInterval.WeekOfYear, m_CurrentWeekStartDate, Microsoft.VisualBasic.FirstDayOfWeek.Monday, FirstWeekOfYear.FirstFourDays)

    m_CurrentWeekNoYear = CurrentWeekNo.ToString("D2") & "-" & CurrentWeekStartYear.ToString
    m_CurrentYearWeekNo = CurrentWeekStartYear.ToString & "-" & CurrentWeekNo.ToString("D2")


    m_PreviousWeekStartDate = DateAdd(DateInterval.Day, -7, m_CurrentWeekStartDate)

    m_PreviousWeekStartYear = DatePart(DateInterval.Year, m_PreviousWeekStartDate)

    m_PreviousWeekNo = DatePart(DateInterval.WeekOfYear, m_PreviousWeekStartDate, Microsoft.VisualBasic.FirstDayOfWeek.Monday, FirstWeekOfYear.FirstFourDays)

    m_PreviousWeekNoYear = PreviousWeekNo.ToString("D2") & "-" & PreviousWeekStartYear.ToString
    m_PreviousYearWeekNo = PreviousWeekStartYear.ToString & "-" & PreviousWeekNo.ToString("D2")

End Sub

对于给定日期dt,从m_CurrentWeekNoYear返回的值的一些示例

• 2015年7月20日 - > 30-2015正确
• 09/03/2015 - > 11-2015正确
• 29/12/2014 - > 53-2014错误应该是01-2015 ...使用02/01/2015也给了53-2014
• 05/01/2015 - > 01-2015不正确应该是02-2015 ...使用09/01/2015也给了01-2015
• 30/12/2013 - > 53-2013错误应该是01-2014 ...使用04/01/2014也给了53-2013
• 06/01/2014 - > 02-2014正确

然后,当我到达一年,实际上有53周它的工作.

• 28/12/2009 - > 53-2009正确
• 04/01/2010 - > 01-2010正确

我出错的任何想法？

Answer 1

众所周知，使用 DateAdd 和 DatePart 在处理这类事情时很糟糕，但问题是 .NET 计算周数的方式存在（错误）不一致。

有关详细信息，请参阅此页面Microsoft .Net 中的 ISO 8601 一年中的周格式

“具体来说，ISO 8601 总是有 7 天的周。如果一年的第一个部分周不包含星期四，则将其算作上一年的最后一周。同样，如果上一年的最后一周不包含星期四，则将其算作上一年的最后一周。包含星期四，然后将其[原文如此]视为明年的第一周。GetWeekOfYear() 具有第一个行为，但没有第二个行为”

这是我编写的一些代码，用于根据定义获取 ISO 8601 周数：

“根据 ISO-8601 标准的周数，从星期一开始的周。一年中的第一周是包含该年第一个星期四的周（=“第一个 4 天周”）。一年中最高的周数是52 或 53。”

''' <summary>
''' Finds the ISO 8601 Week Number based on a given date
''' </summary>
''' <param name="dateValue"></param>
''' <returns></returns>
''' <remarks>ISO 8601 Specifies Monday as the First Day of the week and week one defined as the First Four Day Week</remarks>
Public Shared Function GetIso8601WeekOfYear(ByVal dateValue As DateTime) As Integer
    Return GetWeekOfYear(dateValue, DayOfWeek.Sunday, CalendarWeekRule.FirstFourDayWeek)
End Function

它使用以下更通用的方法来根据 ISO 标准的具体情况查找一年中的第几周：

'Need a calendar - Culture's irrelevent since we specify start day of week
Private Shared cal As Calendar = CultureInfo.InvariantCulture.Calendar


''' <summary>
''' Returns the week number of the year based on the Last Day of the Week and the Week One Rule
''' </summary>
''' <param name="dateValue">The date to find the week number for</param>
''' <param name="lastDayOfWeek">The last day of the week</param>
''' <param name="rule">The Definition of Week One</param>
''' <returns>An integer specifying the week number in the year</returns>
''' <remarks></remarks>
Public Shared Function GetWeekOfYear(ByVal dateValue As DateTime, ByVal lastDayOfWeek As DayOfWeek, ByVal rule As CalendarWeekRule) As Integer
    'There is a bug in the .NET framework where some dates at the end of the year return the incorrect week number so to find the correct value we need to cheat.  
    'Find the DayOfWeek that represents the first day based on the last day
    Dim firstDayOfWeek As DayOfWeek = lastDayOfWeek.Increment
    'Find the date of the last day of the week, this ensures that we get the correct week number value
    dateValue = GetWeekendingDate(dateValue, lastDayOfWeek)
    'Return the value of the week for the last day of the week
    Return cal.GetWeekOfYear(dateValue, rule, firstDayOfWeek)
End Function


''' <summary>
''' Finds the week ending date for a specified date value
''' </summary>
''' <param name="dateValue">The date to find the week ending date for</param>
''' <param name="lastDayOfWeek">The last day of the week</param>
''' <returns>A date value that is the last day of the week that contains the specified dateValue</returns>
''' <remarks></remarks>
Public Shared Function GetWeekendingDate(ByVal dateValue As DateTime, ByVal lastDayOfWeek As DayOfWeek) As DateTime
    'Find out how many days difference from the date we are testing to the end of the week
    Dim dayOffset As Integer = lastDayOfWeek - cal.GetDayOfWeek(dateValue)
    If dayOffset < 0 Then dayOffset += 7
    'Add days to the test date so that it is the last day of the week
    Return dateValue.AddDays(dayOffset)
End Function

此代码使用此扩展方法，该方法可以快速查找以查找一周的第二天：

<Extension()>
Public Function Increment(ByVal aDay As DayOfWeek) As DayOfWeek
    Select Case aDay
        Case DayOfWeek.Sunday : Return DayOfWeek.Monday
        Case DayOfWeek.Monday : Return DayOfWeek.Tuesday
        Case DayOfWeek.Tuesday : Return DayOfWeek.Wednesday
        Case DayOfWeek.Wednesday : Return DayOfWeek.Thursday
        Case DayOfWeek.Thursday : Return DayOfWeek.Friday
        Case DayOfWeek.Friday : Return DayOfWeek.Saturday
        Case DayOfWeek.Saturday : Return DayOfWeek.Sunday
        Case Else : Return Nothing
    End Select
End Function

我用这个尝试了你的测试用例并得到以下结果：

Debug.WriteLine(GetIso8601WeekOfYear(Date.Parse("20/07/2015"))) '30
Debug.WriteLine(GetIso8601WeekOfYear(Date.Parse("09/03/2015"))) '11
Debug.WriteLine(GetIso8601WeekOfYear(Date.Parse("29/12/2014"))) '1
Debug.WriteLine(GetIso8601WeekOfYear(Date.Parse("05/01/2015"))) '2
Debug.WriteLine(GetIso8601WeekOfYear(Date.Parse("30/12/2013"))) '1
Debug.WriteLine(GetIso8601WeekOfYear(Date.Parse("06/01/2014"))) '2