DOCUMENT:Q200299  31-JUL-2001  [vbwin]
TITLE   :BUG: Format or DatePart Functions Can Return Wrong Week Number
PRODUCT :Microsoft Visual Basic for Windows
PROD/VER:WINDOWS:4.0,5.0,6.0
OPER/SYS:
KEYWORDS:kbDateTime kbEurope KbVBA kbVBp400bug kbVBp500bug kbVBp600bug kbDateFormat kbGrpDSVB kb

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The information in this article applies to:

 - Microsoft Visual Basic Learning Edition for Windows, version 6.0 
 - Microsoft Visual Basic Professional Edition for Windows, version 6.0 
 - Microsoft Visual Basic Enterprise Edition for Windows, version 6.0 
 - Microsoft Visual Basic Control Creation Edition for Windows, version 5.0 
 - Microsoft Visual Basic Learning Edition for Windows, version 5.0 
 - Microsoft Visual Basic Professional Edition for Windows, version 5.0 
 - Microsoft Visual Basic Enterprise Edition for Windows, version 5.0 
 - Microsoft Visual Basic Standard Edition, 32-bit, for Windows, version 4.0 
 - Microsoft Visual Basic Professional Edition, 32-bit, for Windows, version 4.0 
 - Microsoft Visual Basic Enterprise Edition, 32-bit, for Windows, version 4.0 
 - Microsoft Visual Basic for Applications versions 5.0, 6.0 
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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 this to be 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.

(c) Microsoft Corporation 2000, All Rights Reserved.
Contributions by Chris E. Jolley, Microsoft Corporation


Additional query words:

======================================================================
Keywords          : kbDateTime kbEurope KbVBA kbVBp400bug kbVBp500bug kbVBp600bug kbDateFormat kbGrpDSVB kbDSupport 
Technology        : kbVBSearch kbAudDeveloper kbZNotKeyword6 kbZNotKeyword2 kbVB500Search kbVB600Search kbVBA500Search kbVBA600Search kbVBA500 kbVBA600 kbVB500 kbVB600 kbVB400Search kbVB400 kbVBASearch kbZNotKeyword3
Version           : WINDOWS:4.0,5.0,6.0
Issue type        : kbbug
Solution Type     : kbpending

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