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Public Function ISO_WeekYearNumber( _
ByVal datDate As Date, _
Optional ByRef intYear As Integer, _
Optional ByRef bytWeek As Byte) _
' Calculates and returns year and week number for date datDate according to the ISO 8601:1988 standard.
' Optionally returns numeric year and week.
' 1998-2007, Gustav Brock, Cactus Data ApS, CPH.
' May be freely used and distributed.
Const cbytFirstWeekOfAnyYear As Byte = 1
Const cbytLastWeekOfLeapYear As Byte = 53
Const cbytMonthJanuary As Byte = 1
Const cbytMonthDecember As Byte = 12
Const cstrSeparatorYearWeek As String = "W"
Dim bytMonth As Byte
Dim bytISOThursday As Byte
Dim datLastDayOfYear As Date
intYear = Year(datDate)
bytMonth = Month(datDate)
bytWeek = DatePart("ww", datDate, vbMonday, vbFirstFourDays)
If bytWeek = cbytLastWeekOfLeapYear Then
bytISOThursday = Weekday(vbThursday, vbMonday)
datLastDayOfYear = DateSerial(intYear, cbytMonthDecember, 31)
If Weekday(datLastDayOfYear, vbMonday) >= bytISOThursday Then
' OK, week count of 53 is caused by leap year.
' Correct for Access97/2000+ bug.
bytWeek = cbytFirstWeekOfAnyYear
' Adjust year where week number belongs to next or previous year.
If bytMonth = cbytMonthJanuary Then
If bytWeek >= cbytLastWeekOfLeapYear - 1 Then
' This is an early date of January belonging to the last week of the previous year.
intYear = intYear - 1
ElseIf bytMonth = cbytMonthDecember Then
If bytWeek = cbytFirstWeekOfAnyYear Then
' This is a late date of December belonging to the first week of the next year.
intYear = intYear + 1
ISO_WeekYearNumber = CStr(intYear) & cstrSeparatorYearWeek & Format(bytWeek, "00")
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Group By ISO_WeekYearNumber([YourDateField])
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