PRB: Date Comparison Can fail Using Arithmetic Operators (194894)


The information in this article applies to:

  • Microsoft Visual Basic Learning Edition for Windows 5.0
  • Microsoft Visual Basic Learning Edition for Windows 6.0
  • Microsoft Visual Basic Professional Edition for Windows 5.0
  • Microsoft Visual Basic Professional Edition for Windows 6.0
  • Microsoft Visual Basic Enterprise Edition for Windows 5.0
  • Microsoft Visual Basic Enterprise Edition for Windows 6.0
  • Microsoft Visual Basic for Applications 5.0
  • Microsoft Visual Basic Standard Edition for Windows 4.0
  • Microsoft Visual Basic Professional Edition for Windows 4.0
  • Microsoft Visual Basic Enterprise Edition for Windows 4.0
This article was previously published under Q194894

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:

130514 : ACC: Storing, Calculating, and Comparing Date/Time Data


42980 : (Complete) Tutorial to Understand IEEE Floating-Point Errors

Modification Type:MajorLast Reviewed:6/29/2004
Keywords:kbcode kbDateTime kbprb KB194894
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