When determining the week number of a date according to the ISO 8601 standard, the underlying function call to the Oleaut32.dll file mistakenly returns week 53 instead of week 1 for the last Monday in certain years.
The ISO 8601 standard is used extensively in Europe and includes the following:
ISO 8601 "Data elements and interchange formats - Information interchange - Representation of dates and times"
ISO 8601 : 1988 (E) paragraph 3.17:
"week, calendar: A seven day period within a calendar year, starting
on a Monday and identified by its ordinal number within the year;
the first calendar week of the year is the one that includes the
first Thursday of that year. In the Gregorian calendar, this is
equivalent to the week which includes 4 January."
This can be implemented by applying these rules for Calendar weeks:
•
A year is divided into either 52 or 53 calendar weeks.
•
A calendar week has 7 days. Monday is day 1, Sunday is day 7.
•
The first calendar week of a year is the one containing at least 4 days.
•
If a year is not concluded on a Sunday, either its 1-3 last days belong to next year's first calendar week or the first 1-3 days of next year belong to the present year's last calendar week.
•
Only a year starting or concluding on a Thursday has 53 calendar weeks.
In Visual Basic and Visual Basic for Applications, all date functionality, except for the DateSerial function, comes from calls to the Oleaut32.dll file. Because both the Format() and DatePart() functions can return the calendar week number for a given date, both are affected by this bug. To avoid this problem, you must use the alternative code that this article provides.
Start a Standard EXE project in Visual Basic. Form1 is created by default.
2.
Add two CommandButtons to Form1.
3.
Paste the following code into the code window of Form1:
Option Explicit
Private Sub Command1_Click()
' This code tests a "problem" date and the days around it
Dim DateValue As Date
Dim i As Integer
Debug.Print " Format function:"
DateValue = #12/27/2003#
For i = 1 To 4 ' examine the last 4 days of the year
DateValue = DateAdd("d", 1, DateValue)
Debug.Print "Date: " & DateValue & " Day: " & _
Format(DateValue, "ddd") & " Week: " & _
Format(DateValue, "ww", vbMonday, vbFirstFourDays)
Next i
End Sub
Private Sub Command2_Click()
' This code lists all "Problem" dates within a specified range
Dim MyDate As Date
Dim Years As Long
Dim days As Long
Dim woy1 As Long
Dim woy2 As Long
Dim ToPrint As String
For Years = 1850 To 2050
For days = 0 To 3
MyDate = DateSerial(Years, 12, 28 + days)
woy1 = Format(MyDate, "ww", vbMonday, vbFirstFourDays)
woy2 = Format(MyDate, "ww", vbMonday, vbFirstFourDays)
If woy2 > 52 Then
If Format(MyDate + 7, "ww", vbMonday, vbFirstFourDays) = 2 Then _
woy2 = 1
End If
If woy1 <> woy2 Then
ToPrint = MyDate & String(13 - Len(CStr(MyDate)), " ")
ToPrint = ToPrint & Format(MyDate, "dddd") & _
String(10 - Len(Format(MyDate, "dddd")), " ")
ToPrint = ToPrint & woy1 & String(5 - Len(CStr(woy1)), " ")
ToPrint = ToPrint & woy2
Debug.Print ToPrint
End If
Next days
Next Years
End Sub
4.
Hold down the CTRL key and press the G key to open the Immediate window.
5.
Run the project, click on Command1, and note the following results in the Immediate window:
Note that with this format, all weeks start with Monday, so that 12/29/2003 should be considered the start of Week 1 and not part of Week 53.
6.
Click on Command2 to see a list of dates in the specified range that experience this problem. The list includes the date, Week day (always Monday), the Week # returned by Format (53), and the Week number it should return (1.) For example:
If you use the Format or DatePart functions, you need to check the return value and, when it is 53, run another check and force a return of 1, if necessary. This code sample demonstrates one way to do this:
Function WOY (MyDate As Date) As Integer ' Week Of Year
WOY = Format(MyDate, "ww", vbMonday, vbFirstFourDays)
If WOY > 52 Then
If Format(MyDate + 7, "ww", vbMonday, vbFirstFourDays) = 2 Then WOY = 1
End If
End Function
You can avoid using these functions to determine Week number by writing code that implements the ISO 8601 rules described above. The following example demonstrates a replacement function to return the Week number.
Start a Standard EXE project in Visual Basic. Form1 is created by default.
2.
From the Project menu, add a new Module, and then paste in the following code:
Option Explicit
Function WeekNumber(InDate As Date) As Integer
Dim DayNo As Integer
Dim StartDays As Integer
Dim StopDays As Integer
Dim StartDay As Integer
Dim StopDay As Integer
Dim VNumber As Integer
Dim ThurFlag As Boolean
DayNo = Days(InDate)
StartDay = Weekday(DateSerial(Year(InDate), 1, 1)) - 1
StopDay = Weekday(DateSerial(Year(InDate), 12, 31)) - 1
' Number of days belonging to first calendar week
StartDays = 7 - (StartDay - 1)
' Number of days belonging to last calendar week
StopDays = 7 - (StopDay - 1)
' Test to see if the year will have 53 weeks or not
If StartDay = 4 Or StopDay = 4 Then ThurFlag = True Else ThurFlag = False
VNumber = (DayNo - StartDays - 4) / 7
' If first week has 4 or more days, it will be calendar week 1
' If first week has less than 4 days, it will belong to last year's
' last calendar week
If StartDays >= 4 Then
WeekNumber = Fix(VNumber) + 2
Else
WeekNumber = Fix(VNumber) + 1
End If
' Handle years whose last days will belong to coming year's first
' calendar week
If WeekNumber > 52 And ThurFlag = False Then WeekNumber = 1
' Handle years whose first days will belong to the last year's
' last calendar week
If WeekNumber = 0 Then
WeekNumber = WeekNumber(DateSerial(Year(InDate) - 1, 12, 31))
End If
End Function
Function Days(DayNo As Date) As Integer
Days = DayNo - DateSerial(Year(DayNo), 1, 0)
End Function
3.
Add a CommandButton to Form1.
4.
Paste the following code into the code window of Form1:
Private Sub Command1_Click()
Dim DateValue As Date, i As Integer
Debug.Print " WeekNumber function:"
DateValue = #12/27/2003#
For i = 1 To 4 ' examine the last 4 days of the year
DateValue = DateAdd("d", 1, DateValue)
Debug.Print "Date: " & DateValue & " Day: " & _
Format(DateValue, "ddd") & " Week: " & WeekNumber(DateValue)
Next i
End Sub
5.
Hold down the CTRL key and press the G key to open the Immediate window.
6.
Run the project and click on Command1 to see the following results in the Immediate window:
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