Microsoft KB Archive/241728

= PRB: Using 2-Digit Years with IsDate May Produce Unexpected Results =

Article ID: 241728

Article Last Modified on 6/28/2004

-

APPLIES TO


 * 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 5.0 Learning Edition
 * Microsoft Visual Basic 6.0 Learning Edition
 * Microsoft Visual Basic 5.0 Professional Edition
 * Microsoft Visual Basic 6.0 Professional Edition
 * Microsoft Visual Basic 5.0 Enterprise Edition
 * Microsoft Visual Basic 6.0 Enterprise Edition
 * Microsoft Visual Basic for Applications 5.0
 * Microsoft Visual Basic for Applications 6.0

-



This article was previously published under Q241728



SYMPTOMS
The IsDate function may return unexpected results if passed a date which contains a 2-digit year.



CAUSE
The VBA date functions IsDate, Format, CDate, and CVDate utilize a function found in OLE Automation (OleAut32.dll). This function searches all possible date formats by tokenizing each of the separated values in the string representing the date and returns a Boolean value indicating whether the input can be represented as a Date.

This is important to remember when using the function to interpret a date that contains a 2 digit year. Different Locales use various date formats (that is, mm/dd/yy, yy/mm/dd, "DD MMM YY", "YY MMM DD", and so forth) and therefore the function tries the digits in all positions until the function has found a valid date or exhausted all possibilities.

Checking whether February 29th is a valid date for a specific year, is one example of where you may get unexpected results when passing the IsDate function a date that contains a 2-digit year. To be more specific, passing the IsDate function an ambiguous date such as "29-FEB-01", will result in IsDate checking all available date formats and return TRUE because February 1, 2029 is a valid date. However, when the fully qualified year is passed in as "29-Feb-2001", then IsDate can determine that this is an invalid Date, and therefore will return FALSE.



RESOLUTION
Create a wrapper function around the IsDate function to convert the date to a 4-digit year before passing the converted date to the IsDate function.



STATUS
This behavior is by design.



MORE INFORMATION
The functions in OleAut32.dll use a standard "sliding year" so that, by default, all 2-digit years in the range 0 - 29 are considered to be in the 2000s and those in the range 30 - 99 are in the 1900s. This can be easily overridden with the following wrapper function.

The documentation for the IsDate function defines it's designed behavior.

IsDate(expression)

The required expression is a Variant containing a date expression or string expression recognizable as a date or time.

Steps to Reproduce Behavior
 Start a new VB Standard EXE Project. Form1 is created by default. Place a CommandButton (Command1) on Form1.  Paste the following code into the declarations section of Form1.

Private Sub Command1_Click Dim bIsDate As Boolean Dim sDate As String

sDate = "29 FEB 01" bIsDate = IsDate(sDate) If bIsDate Then MsgBox "Valid Date Found : " & Format(sDate, "mm/dd/yyyy") Else MsgBox sDate & " Is Not a Valid Date" End If  End Sub  Select F5 to run the project and click Command1. The following message will appear:

Valid Date Found : 02/01/2029



Steps to Avoid the Problem
  Replace the earlier code in the Form1 module with the following code that includes the use of a wrapper function: Private Sub Command1_Click Dim bIsDate As Boolean Dim sDate As String

sDate = ConvertYear("29 FEB 01") bIsDate = IsDate(sDate) If bIsDate Then MsgBox "Valid Date Found : " & Format(sDate, "mm/dd/yyyy") Else MsgBox sDate & " Is Not a Valid Date" End If  End Sub

Private Function ConvertYear(sDate As String) As String Dim sYear As String

' This function currently uses the same sliding year as OleAut32.dll, ' but can be customized to fit the needs of the application.

sYear = Right(sDate, 2) If Val(sYear) <= 29 Then ConvertYear = Left(sDate, 6) & " 20" & sYear Else ConvertYear = Left(sDate, 6) & " 19" & sYear End If

End Function </li> Run the project and select Command1. As expected, the following message will appear:

29 FEB 2001 Is Not a Valid Date

</li></ol>

Keywords: kbdatetime kbprb KB241728

-

[mailto:TECHNET@MICROSOFT.COM Send feedback to Microsoft]

© Microsoft Corporation. All rights reserved.