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Why You Should Enter Dates With Four-Digit Years Whenever Possible

Why you should tell your programs which century your dates fall into, not the other way around!
How do your programs decide to which century to assign a date with a two-digit year?
What if a program doesn't allow you to enter dates with four-digit years?

Related documents

Checking your data for Y2K problems
(article appearing in the November-December 1999 issue of Berkeley Computing and Communications )
Data Files: Finding & Resolving Y2K Problems
The Windows Automation Libraries Determine How Dates Entered With Two-Digit Years Are Handled
Commercial Off-the-Shelf Applications: Finding & Resolving Y2K Problems

Why you should tell your programs which century your dates belong to, not the other way around!

Many of us have undoubtedly adopted the common habit of entering dates containing just two digits for the year (ending in values like "98" or "/98") into our application programs. For example, we may have typed dates with two-digit years into spreadsheet cells or entered such dates into our database or client/server programs.

As we begin the transition to the 21st Century, this habit of entering dates in a "shorthand" format with only two-digit years can cause problems. When you enter a date with only a two-digit year, such as 05/13/29, many application programs will essentially take an educated guess at what century -- the 20th or 21st -- "you probably meant." Because automated rules can never perfectly substitute for human judgment, sometimes your programs may guess wrong. If you don't notice when your programs have done this, you run the risk that some dates will be assigned to centuries other than you intended. In some cases, this could significantly impact the integrity of your data.

For this reason we recommend that, whenever possible, you enter dates with full, four-digit years (e.g. 05/13/2029) into spreadsheets, database files, and other data files. It's preferable that you tell your programs what century your dates belong to, rather than leaving it up to your programs to guess at this. (You can also enforce this policy: if you should require that your most critical dates always be entered with full, four-digit years, some database programs - and certain other types of programs - offer "field validation" or "input mask" options that you can use for this purpose.)

How do your programs decide to which century to assign a date with a two-digit year?

Some programs use a "windowing" rule which assigns dates to either the 20th or 21st Centuries by comparing their two-digit year to a "pivot year": dates with two-digit years higher than the pivot year are assigned to the 20th Century, and dates with two-digit years lower than (or equal to) the pivot year are assigned to the 21st Century. By contrast, some other, typically older, applications simply assume that all two-digit-year dates fall into the 1900s.

The following are some representative examples of how programs differ in their handling of dates with two-digit years:

Some of the year ranges and annotations above are taken from Seven Things to Know: Desktop Applications and the Year 2000, A Guide for Microsoft and Lotus Spreadsheet and Database Analysis, , Copyright (c) 1998, IST Development, Inc.

(The examples above describe how these programs handle two-digit year dates entered from the keyboard directly into data files. Some of these programs handle imported dates, or dates used in other program functions, in different ways than described above.)

The methods your programs use to decide to which century to assign a date with a two-digit year may vary depending on as many as four different factors:

• Which program you're using.
  
  Different programs use different methods for assigning dates with two-digit years to the 20th or 21st centuries.
  
  
• Which version of the program you're using.
  
  For example, if you enter the date 05/13/29, some earlier versions of Microsoft Excel will interpret this date as May 13, 1929, while the latest versions of Excel will interpret this date as May 13, 2029.
  
  
• The current year.
  
  A few programs may use a windowing technique which will try to intelligently assign dates with two-digit years to the correct century based, in part, on the current year.
  
  For example, as described in FileMaker, Inc.'s Year 2000 Readiness Disclosure: FileMaker Products , the FileMaker Pro database program, in versions 3.0 and later, changes the way it interprets dates with two-digit years depending on whether the current year falls within the last ten years of a century, within the first ten years of the next century, or within a period which is over ten years away from a century transition.
  
  This means that, beginning with the year 2000, FileMaker Pro will handle the interpretation of dates with two-digit years differently from the way it handled such dates during the decade of the 1990s.
  
  
• Which versions of date-related operating system files are installed on your computer.
  
  Many Windows programs rely on programming routines provided by Microsoft's OLE Automation Libraries - a set of four operating system files - to decide how to assign dates with two-digit years to the 20th or 21st centuries. Older versions of these files always assigned dates with two-digit years to the 1900s, while newer versions use updated rules to decide whether such dates should be assigned to the 1900s or 2000s.
  
  If you reinstall the Windows operating system; apply Service Packs or other Windows updates; or even install certain application programs, such as the Microsoft Internet Explorer Web browser, this can cause different versions of the OLE Automation Libraries files to be installed, which can in turn change the way that some of your Windows programs handle dates with two-digit years.
  
  To read more about this, see The Windows Automation Libraries Determine How Dates Entered With Two-Digit Years Are Handled .

Worse, some (particularly older) programs are permanently stuck in the 1900s when it comes to dates with two-digit years, always interpreting such dates as falling into the 20th century. As a result, these programs would interpret "01/31/98" as January 31, 1998, as you might expect, but they would also interpret "01/31/00" as January 31, 1900, rather than as January 31, 2000.

The inability to be certain how your programs will assign dates with two-digit years to the 20th or 21st centuries makes a compelling case for always entering dates with four-digit years.

What if a program doesn't allow you to enter dates with four-digit years?

Some Y2K non-compliant application programs simply may not allow you to enter dates with four-digit years. Commercial Off-the-Shelf Applications: Finding & Resolving Y2K Problems describes what you can do if you should encounter one of these programs.

A few programs may require that, in order to enter dates with four digit years, you first change your operating system's default date format to use four-digit years. Changing Your Operating System's Default Date Format To Use Four-Digit Years describes how to do so.