Reasoning
For organizations and institutions with data that is only decades old, a "date windowing" solution was considered easier and more economical than the massive conversions and testing required when converting two-digit years into four-digit years.Windowing methods
There are three primary methods used to determine the date window: * Fixed pivot year: simplest to code, works for most business dates. * Sliding pivot year: determined by subtracting some constant from the current year, typically used for birth dates. * Closest date: Three different interpretations (last century, this century, and next century) are compared to the current date, and the closest date is chosen from the three.FOCUS
Examples
Below is a simple example of fixed date windowing COBOL code, often used to figure the century for ordinary business dates.See also
* Serial number arithmetic, a form of windowing for sequential countersReferences
{{Reflist, 2, refs= {{Cite web , url=http://x109.healton.net/y2k/yrexamples.html#_GoodGuys.Pivot , title=The Best of Dates, The Worst Of Dates , last=Healton , first=Gilbert , date=2014-07-12 , format=html , archive-url=https://web.archive.org/web/20200213231310/http://x109.healton.net/y2k/yrexamples.html#_GoodGuys.Pivot.Intro , archive-date=2020-02-13 , access-date=2020-02-13 , quote=A pivot year, also known as date windowing, takes a two-digit year and expands it to determine which century the year is in. Typically the year is converted to either a full four-digit year or into the year-1900 format, as appropriate to the application at hand. , df=dmy-all {{cite journal , author=Raymond B. Howard, title=The Case for Windowing: Techniques That Buy 60 Years, journal=Year/2000 Journal , issue=Mar/Apr 1998 , quote=Windowing is a long-term fix that should keep legacy systems working fine until the software is redesigned... {{Cite web , url=https://grumpyoldastronomer.com/y2kpivot.htm , title=The Y2K Pivot Year Problem , last=McNish , first=Larry , date=2019-12-30 , archive-url=https://web.archive.org/web/20200115130255/https://grumpyoldastronomer.com/y2kpivot.htm , archive-date=2020-01-15 , access-date=2020-02-13 , quote=The choice of the actual pivot year was made by companies based on their historical records. It was in no way an industry standard, nor was this approach in any way professionally recommended. But it would work for a while. , df=dmy-all {{Cite web , url=https://www.appx.com/common/documents/manuals/appx/sysadmin/Chapter_1_Orientation_to_the_Manual/Chapter_1__Orientation_to_t.htm , title=Chapter 1: Pivot Year , date=2003 , website=APPX Software, Inc. , format=html , archive-url=https://web.archive.org/web/20120922230258/https://www.appx.com/common/documents/manuals/appx/sysadmin/Chapter_1_Orientation_to_the_Manual/Chapter_1__Orientation_to_t.htm , archive-date=2012-09-22 , access-date=2020-02-13 , quote=By defining a pivot year, you direct APPX to determine what century a particular year belongs to. For example, if the pivot year is set to 35, then any date field with a year greater than 35 will be assumed by APPX to have a century value of 19. Any date field with a year less than or equal to the pivot year will be assumed by APPX to have a century value of 20. , df=dmy-all {{Cite web , url=http://edition.cnn.com/TECH/computing/9902/09/testy2k.idg/index.html , title=COMPUTING Spot & fix Y2K problems in Windows 9x & NT , last=Livingston , first=Brian , date=1999-02-09 , website= CNN , format=html , archive-url=https://web.archive.org/web/20010517090846/http://edition.cnn.com/TECH/computing/9902/09/testy2k.idg/index.html , archive-date=2001-05-17 , access-date=2020-02-13 , quote=The other common behavior that Windows users will experience involves dates in spreadsheet programs such as Microsoft Excel, Lotus 1-2-3, and Corel Quattro Pro. If you have typed all your dates with four-digit years (such as 1/1/1999 and 1/1/2000), you should be fine. But if you have typed two-digit years (such as 1/1/29), you may be surprised at the way different programs handle the date. , df=dmy-all {{Cite web , url=https://www.onlineathens.com/stories/031699/new_y2k.shtml , title=Temporary Y2K fix may last only a generation , last=Bridis , first=Ted , date=1999-03-16 , website=Online Athens , format=html , archive-url=https://web.archive.org/web/20160304074157/https://www.onlineathens.com/stories/031699/new_y2k.shtml , archive-date=2016-03-04 , access-date=2020-02-13 , quote=Using windowing, programmers instruct software to guess the century for dates that fall within a specific "window" of time, such as the next three decades. The computer interprets the year based on a future so-called hinge date, or pivot, that programmers choose arbitrarily , df=dmy-all Units of time