Format or DatePart functions can return wrong week number for last Monday in Year

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.

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. Open the Visual Basic project within an Office application (Alt + F11).

  2. From the Project menu, add a new module.

  3. Paste the following code into the module:

    Option Explicit
    
    Public Function Test1()
    ' 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 Function
    
    Public Function Test2()
    ' 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 Function
    
  4. Use (Ctrl + G) to open the Immediate Window if it's not already open.

  5. Type ?Test1 in the Immediate window and hit Enter, 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. Type ?Test2 in the Immediate window and hit Enter 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. 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. Open the Visual Basic project within an Office application (Alt + F11).

  2. From the Project menu, add a new module.

  3. Paste the following code into the module:

    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
    
    Public Function Test3()
     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 Function
    
  4. Use (Ctrl + G) to open the Immediate Window if it's not already open.

  5. Type ?Test3 in the Immediate window and hit Enter, note 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.