Q197431: PRB: Year Returned by DateSerial may be Unanticipated

Article: Q197431
Product(s): Microsoft Visual Basic for Windows
Version(s): WINDOWS:5.0,6.0
Operating System(s): 
Keyword(s): kbGrpDSVB
Last Modified: 11-JAN-2001

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The information in this article applies to:

- Microsoft Visual Basic Learning Edition for Windows, versions 5.0, 6.0 
- Microsoft Visual Basic Professional Edition for Windows, versions 5.0, 6.0 
- Microsoft Visual Basic Enterprise Edition for Windows, versions 5.0, 6.0 
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SYMPTOMS
========

The DateSerial function can seem to return unanticipated results using years
represented by numeric expressions. For example, the following evaluates to
1/1/100 and not 1/1/00 or 1/1/2000 because 99 + 1 or 100 is a valid full size
date:

     DateSerial(99 + 1, 1, 1)

CAUSE
=====

Visual Basic documentation gives the following rule concerning year arguments
used by the DateSerial function:

  For the year argument, values between 0 and 29, inclusive, are interpreted as
  the years 2000-2029. Values between 30 and 99, inclusive, are interpreted as
  the years 1930-1999.

This rule only applies to year arguments represented with 1 or 2 digits. This can
sometimes produce results that may not be anticipated. For example, in the
following line of code the year is actually 100, which is a valid 3 digit value
in the range of 100 to 9999:

  DateSerial(99 + 1, 1, 1)

Therefore, DateSerial correctly returns the date 1/1/100. In this case DateSerial
will not return 1/1/00 or 1/1/2000. This type of behavior can occur whenever the
numeric expression in the year argument evaluates to a year having more than 2
digits.

RESOLUTION
==========

There are several ways to get the desired result (1/1/2000) using years passed
as numeric expressions. One way would be to use DateSerial with the full 4 digit
year. This example returns 1/1/00:

  DateSerial(1999 + 1, 1, 1)

DateSerial can also be used in conjunction with the year function. This example
also returns 1/1/00:

  DateSerial(Year(#1/1/99#)+1, Month(#1/1/99#), Day(#1/1/99#))

In addition to the DateSerial function, the DateAdd or DateDiff functions can
also be used. The following example also returns 1/1/00:

  DateAdd("yyyy", 1, "1/1/99")

MORE INFORMATION
================

Steps to Reproduce Behavior
---------------------------

1. Create a new Standard EXE Project in Visual Basic. Form1 is created by
  default.

2. Add three command buttons and one text box to the default form (Form1).

3. Add the following code to the General Declarations of the form:

        Dim k As Integer
        Private Sub Command1_Click()
           k = MsgBox(DateAdd("yyyy", 1, Text1), vbOKOnly, _
           "DateAdd")
        End Sub

        Private Sub Command2_Click()
           k = MsgBox(DateSerial(Year(Text1) + 1,Month(Text1), _
           Day(Text1)), OKOnly, "DateSerial with Year")
        End Sub

        Private Sub Command3_Click()
           k = MsgBox(DateSerial(Right(Text1, 2) + 1,Month(Text1), _
           Day(Text1)), vbOKOnly, "DateSerial without Year")
        End Sub

        Private Sub Form_Load()
           Text1 = "1/1/99"
        End Sub

4. Run the program and try clicking the different buttons.

Notice that Command1 and Command2 display the date 1/1/00 and Command3 displays
the date 1/1/100.

======================================================================
Keywords          : kbGrpDSVB 
Technology        : kbVBSearch kbAudDeveloper kbZNotKeyword6 kbZNotKeyword2 kbVB500Search kbVB600Search kbVBA500 kbVBA600 kbVB500 kbVB600
Version           : WINDOWS:5.0,6.0
Issue type        : kbprb

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