Q194894: PRB: Date Comparison Can fail Using Arithmetic Operators

Article: Q194894
Product(s): Microsoft Visual Basic for Windows
Version(s): WINDOWS:4.0,5.0,6.0
Operating System(s): 
Keyword(s): kbcode kbDateTime KbVBA kbVBp400 kbVBp500 kbVBp600 kbGrpDSVB kbCodeSam
Last Modified: 24-JUL-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 
- Microsoft Visual Basic for Applications version 5.0 
- Microsoft Visual Basic Standard Edition for Windows, version 4.0 
- Microsoft Visual Basic Professional Edition for Windows, version 4.0 
- Microsoft Visual Basic Enterprise Edition for Windows, version 4.0 
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SYMPTOMS
========

Comparing two apparently identical dates using the standard comparison operators
produces anomalous results.

CAUSE
=====

The Visual Basic Date data type is stored internally as a Double (64 bit
floating point). Performing operations on Date variables is subject to the same
rounding problems as any other floating point value. This is true even if most
of the intrinsic functions, such as DateAdd, are used.

RESOLUTION
==========

1. Use the DateDiff intrinsic function to compare date and time values. DateDiff
  rounds the input values to the specified tolerance, eliminating floating
  point errors.

2. Take the difference between two variables of type Date and compare the
  absolute value of the result to a tolerance value such as 10E-11. If the
  difference is less than the tolerance, the values may be considered
  identical.

STATUS
======

This behavior is by design.

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

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

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

2. Add the following code to the General Declarations section of Form1:

        Option Explicit

        Private Sub Form_Load()
           Dim Date1 As Date
           Dim Date2 As Date
           Dim Date3 As Date
           Dim Date4 As Date
           Me.Width = 6500
           Me.Height = 3000
           Me.Show
           Date1 = #10/21/1998 8:00:00 AM#
           Date2 = #10/21/1998 8:20:00 AM#
           Date3 = DateAdd("n", 20#, Date1)
           Date4 = Date1 + TimeSerial(0, 20, 0)
           Print "The results are visually identical..."
           Print
           Print "Date2 = "; Date2
           Print "Date3 = "; Date3
           Print "Date4 = "; Date4
           Print
           Print "but the actual values are not"
           Print
           Print Tab(20), "=", "DateDiff", "Actual Difference"
           Print "Date2 = Date3?", Date2 = Date3,
           Print DateDiff("s", Date2, Date3), Date2 - Date3
           Print "Date2 = Date4?", Date2 = Date4,
           Print DateDiff("s", Date2, Date4), Date2 - Date4
           Print "Date3 = Date4?", Date3 = Date4,
           Print DateDiff("s", Date3, Date4), Date3 - Date4
        End Sub

3. Run the application and examine the results on the form.

REFERENCES
==========

For additional information, please see the following articles in the Microsoft
Knowledge Base:

  Q130514 : ACC: Storing, Calculating, and Comparing Date/Time Data


  Q42980 : (Complete) Tutorial to Understand IEEE Floating-Point Errors

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Keywords          : kbcode kbDateTime KbVBA kbVBp400 kbVBp500 kbVBp600 kbGrpDSVB kbCodeSam 
Technology        : kbVBSearch kbAudDeveloper kbZNotKeyword6 kbZNotKeyword2 kbVB500Search kbVB600Search kbVBA500 kbVB500 kbVB600 kbVB400Search kbVB400 kbVBASearch kbZNotKeyword3
Version           : WINDOWS:4.0,5.0,6.0
Issue type        : kbprb

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