BUG: Format or DatePart Functions Can Return Wrong Week Number for Last Monday in Year

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Article ID: 200299 - View products that this article applies to.
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SYMPTOMS

When you use either the Format or DatePart function to determine the week number for dates using the following syntax:
Format(AnyDate, "ww", vbMonday, vbFirstFourDays)

DatePart("ww", AnyDate, vbMonday, vbFirstFourDays)
				
the last Monday in some calendar years is returned as week 53 when it should be week 1.

CAUSE

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.

RESOLUTION

Use a user-defined function to return the Week number based on the rules for the ISO 8601 standard. An example is included in this article.

STATUS

Microsoft has confirmed that this is a problem in the Oleaut32.dll file.

MORE INFORMATION

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.

Steps to Reproduce Behavior

  1. 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:
       Format function:
    Date: 12/28/03   Day: Sun   Week: 52
    Date: 12/29/03   Day: Mon   Week: 53
    Date: 12/30/03   Day: Tue   Week: 1
    Date: 12/31/03   Day: Wed   Week: 1
    						
    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:
    12/29/1851   Monday    53   1
    12/31/1855   Monday    53   1
    12/30/1867   Monday    53   1
    12/29/1879   Monday    53   1
    12/31/1883   Monday    53   1
    12/30/1895   Monday    53   1
    ...
    					

Workarounds

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.

Step by Step Example

  1. 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:
       WeekNumber function:
    Date: 12/28/03   Day: Sun   Week: 52
    Date: 12/29/03   Day: Mon   Week: 1
    Date: 12/30/03   Day: Tue   Week: 1
    Date: 12/31/03   Day: Wed   Week: 1
    					
    Note that Monday is considered to be Week 1 as it should be.

Properties

Article ID: 200299 - Last Review: June 24, 2004 - Revision: 3.0
APPLIES TO
  • Microsoft Visual Basic 6.0 Learning Edition
  • Microsoft Visual Basic 6.0 Professional Edition
  • Microsoft Visual Basic 6.0 Enterprise Edition
  • Microsoft Visual Basic 5.0 Control Creation Edition
  • Microsoft Visual Basic 5.0 Learning Edition
  • Microsoft Visual Basic 5.0 Professional Edition
  • Microsoft Visual Basic 5.0 Enterprise Edition
  • Microsoft Visual Basic 4.0 Standard Edition
  • Microsoft Visual Basic 4.0 Professional Edition
  • Microsoft Visual Basic 4.0 32-Bit Enterprise Edition
  • Microsoft Visual Basic for Applications 5.0
  • Microsoft Visual Basic for Applications 6.0
Keywords: 
kbbug kbdatetime kbpending KB200299

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