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As a developer, you may be concerned about how your applications will handle dates beyond 12/31/1999. On January 1st of the year 2000, will your programs think it is the year 2000, 1900, or even 1980? The goal of this article is to ensure that your Visual Basic applications will behave properly with dates beyond 12/31/1999.
While all versions of Visual Basic will handle years greater than 1999 (in a four-digit format), passing a two-digit year in a date (such as 7/3/45) forces Visual Basic to assume what century the date corresponds to. Perhaps the best way to explain this behavior would be to give a quick Visual Basic history lesson.
Visual Basic History LessonFor all versions of Visual Basic for Windows (including its predecessors such as Visual Basic for DOS and QuickBasic) prior to and including 3.0, two-digit years were always assumed to be in the 1900s. The code to implement this default was built into each version's run-time library and does not depend on the version of the operating system or the century of the current system date.
Between the development cycles for Visual Basic 3.0 and 4.0, two new entities emerged: Visual Basic for Applications and OLE Automation. Prior to the advent of these technologies, Visual Basic's runtime library contained the code responsible for converting a two-digit year to a four-digit year. OLE Automation exposed a great deal of functionality that other applications could access. Visual Basic for Applications did not need to implement this code; it could make calls to the OLE Automation libraries instead.
Visual Basic 4.0 was developed with this interoperability in mind and began to rely on the OLE Automation libraries to convert two-digit years to four-digit years in most cases. The exception to the rule is the DateSerial function that was implemented in the Visual Basic runtime library because Visual Basic required more functionality than the OLE Automation library could provide at that time.
During the Visual Basic 4.0 development cycle, Microsoft decided that the defaults used in previous versions of Visual Basic were reliable but not necessarily practical. So, a new rule emerged. A two-digit year would be converted to the current century of the system date. Thus, Year(Date("1/1/00")) would evaluate to the current century. This new rule was implemented in the OLE Automation libraries used by Visual Basic 4.0 and Visual Basic for Applications. The Visual Basic 4.0 runtime library also implements the rule for the DateSerial function.
Microsoft later reconsidered and changed the default in the OLE Automation (now simply Automation) libraries as of version 2.20.4049 of OleAut32.dll. This change does not affect 16-bit applications that rely on the Automation libraries, only 32-bit applications. Now, a two-digit year between 00 and 29 (such as 17) is interpreted as 2017 while a two-digit year between 30 and 99 (such as 72) is interpreted as 1972. The new Automation libraries provide the functionality that Visual Basic requires for the DateSerial function. Thus, Visual Basic 5.0 and subsequent releases no longer implement rules for this function in their runtime libraries.
The updated Automation library ships with Internet Explorer version 3.0 and later, Windows NT 3.51 Service Pack 5, Windows NT 4.0, Windows 95 OSR 2, Office 97, Visual Basic 5.0 and other products.
What Does All of This Mean to Developers Who Use Visual Basic?Visual Basic 3.0 and prior versions convert all two-digit years to the 1900s.
Visual Basic 4.0 (16-bit) converts all two-digit years to the century of the current system date. Depending on the function used, Visual Basic converts the date based on defaults in either the 16-bit Automation libraries or the runtime library. The defaults in the 16-bit Automation libraries have not been modified since Visual Basic 4.0 released so the behavior is consistent regardless of which date function is used.
Visual Basic 4.0 (32-bit) converts two-digit years to four-digit years based on the default in the Automation libraries except when using the DateSerial function that converts all two-digit years to the century of the current system date. The 32-bit Automation libraries (OleAut32.dll version 2.10) that shipped when Visual Basic 4.0 was released converted all two-digit years to the century of the current system date. Later 32-bit Automation libraries (OleAut32.dll version 2.20 and later) convert two-digit years to the 1900s if the two-digit year is between 30 and 99. If the two-digit year is between 00 and 29, the date is converted to the 2000s.
Visual Basic versions subsequent to 4.0, convert two-digit years to four- digit years based on the default in the Automation libraries for all date functions. Visual Basic 5.0 shipped with version 2.20.4054 that converts two-digit years to the 1900s if the two-digit year is between 30 and 99. If the two-digit year is between 00 and 29, the date is converted to the 2000s.
What If I Don't Like Those Defaults?You may want to use your own set of rules instead of relying on the defaults native to Visual Basic. For example, you may want to enter only a two-digit year and have 00 to 49 correspond to the years 2000 to 2049 and have 50 to 99 correspond to the years 1950 to 1999.
When accepting a date string from the user, test the format of the string to determine the number of digits entered for the year. According to the rules for this sample application, 1/11/45 is in the year 2045, and not in the year 1945. Within the code for the application, change the string to use the appropriate four-digit year, then convert that date string with the four-digit year into a date variable.
Sample CodeThe following code evaluates the data entered into a textbox named txtDate when you click cmdConvertDate. If the date contains a two-digit year, the date is converted into a four-digit year date according to the sample rule. The code then displays the initial date entered, the full year as converted by the code according to the sample rule, and the full year is converted by the defaults native to Visual Basic. Finally, the date displayed in txtDate is converted to a non-ambiguous date with the appropriate four-digit year.
Once you have developed code to handle your own rules for interpreting two- digit years, you can build that code into a 32-bit custom control with the Control Creation Edition of Visual Basic. For more information on this product and on building your own custom controls, please see the Microsoft Visual Basic Web site at:
This code requires that dates be entered in the mm/dd/yy format, but it could easily be changed to handle a different date format:
Microsoft Visual Basic Help file; Search on: Date and Variant Data Types; IsDate function; CDate and CVDate functions;
(c) Microsoft Corporation 1997, All Rights Reserved. Contributions by David Sceppa, Microsoft Corporation
Article ID: 162718 - Last Review: February 28, 2014 - Revision: 3.3
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