Instead of "Y2K" being just an "end of 1999" problem, there are a number of critical (or sometimes called "trigger") dates to check out in detail to ensure continuity of service in computer systems. The following is a list of critical dates where computerized information systems and embedded systems need to be comprehensively tested for forward and backward date processing to ensure accuracy.
In other words, although this is not an exhaustive listing, be on the lookout for erroneous results around any of these dates.
| Date | Explanation |
|---|---|
| Nov. 2, 1997 | Overflow HP/Apollo Domain Operating System. |
| Jan. 1, 1998 | Ensure that the digits "98" do not trigger an error. |
| Oct. 1, 1998 | Beginning of FY 1999 for Federal Government |
| Dec. 31, 1998 | Ensure that the digits "99" do not trigger an error. |
| Jan. 1, 1999 | First day of 1999. |
| Jan. 1, 1999 | Introduction of electronic version of the Euro. |
| Feb. 4, 1999 | Airlines Begin Accepting reservations in year 2000. |
| Mar. 1999 | Securities Industry Association simulates end-year trading. |
| Apr. 1, 1999 | Begin of fiscal year in Canada, Japan.[2] |
| Apr. 1, 1999 | Begin of fiscal year in New York State.[2] |
| Apr. 6, 1999 | Begin of fiscal year in Great Britain.[1] |
| Apr. 9, 1999 | This date is "9999" or day 99 of year 99 on the Julian Calendar. |
| July 1, 1999 | Begin Federal Fiscal Year in most U.S. states.[2] |
| July 1, 1999 | Event horizon for 6-month forecasting.[2] |
| Aug. 22, 1999 | Week counter in GPS satellites resets. |
| Sep. 1, 1999 | Begin of fiscal year in Texas.[1] |
| Sep. 9, 1999 | This is 9/9/99, a common coding convention. |
| Sep. 23, 1999 | 99 days until the Year 2000. |
| Sep. 30, 1999 | This is the last fiscal rollover prior to end of year. |
| Oct. 1, 1999 | Begin Federal Fiscal Year.[2] |
| Oct. 1, 1999 | Alabama and Michigan fiscal years 2000 begin. |
| Oct. 1, 1999 | Event horizon for 3-month forecasting.[1] |
| Oct. 10, 1999 | First date in the year to mandate eight digit date |
| Nov. 1, 1999 | Event horizon for 1-month forecasting.[1] |
| Dec. 1, 1999 | First monthly plan to look ahead past end of year. |
| Dec. 30, 1999 | Next to last day in year 1999.[1] |
| Dec. 31, 1999 | Last day in year 1999.[1] |
| Dec. 31, 1999 | A "never expire" date on old IBM data tapes. |
| Jan. 0, 2000 | Illegal date - must not occur. |
| Jan. 1, 2000 | First day of the year 2000.[1] |
| Jan. 2, 2000 | First Sunday of the year 2000.[1] |
| Jan. 3, 2000 | First business day in the year 2000, but a holiday.[2] |
| Jan. 4, 2000 | First workday and first banking day in the year 2000.[1] |
| Jan. 6, 2000 | First weekday possibly mistaken for weekend day. |
| Jan. 7, 2000 | First Friday and first payday in the year 2000.[1] |
| Jan. 10, 2000 | First date to require a 7 digit date field (1/10/2000).[1] |
| Jan. 10, 2000 | First day of classes at University of Colorado. |
| Jan. 14, 2000 | First semi-monthly payday. |
| Jan. 28, 2000 | End of first work month.[1] |
| Jan. 17, 2000 | Martin Luther King Day; This is the first Monday holiday in the Year 2000. |
| Jan. 31, 2000 | First end of month; first monthly payday. W2s due. |
| Feb. 28, 2000 | Day before the leap day.[1] |
| Feb. 29, 2000 | Valid date - February has 29 days in the year 2000.[2] |
| Feb. 30, 2000 | Illegal date - must not occur. |
| Mar. 1, 2000 | Ensure date calculations have taken leap year into account. |
| Mar 31, 2000 | End of First Quarter Fiscal year.[1] |
| Apr. 1, 2000 | Possible false change to Daylight Savings Time. |
| Apr. 2, 2000 | First change to Daylight Savings Time after end of year. |
| Apr. 3, 2000 | First business day after fiscal quarter. |
| Apr, 6, 2000 | Beginning of United Kingdom's Fiscal Year 2000 |
| Apr. 14, 2000 | Last business day for U.S.A. 1999 tax transactions. |
| Apr. 15, 2000 | 1999 tax filing deadline for U.S.A. |
| Apr. 17, 2000 | U.S. personal income tax due.[1] |
| June 6, 2000 | End of Second Quarter Fiscal year.[1] |
| June 6, 2000 | End of Australian tax year.[1] |
| July 1, 2000 | Beginning of Fiscal Year 2001 for most States |
| Oct. 1, 2000 | This is the first 7-digit date with a 2-digit month value. (10/1/2000) |
| Oct. 10, 2000 | First date to require an 8 digit date field (10/10/2000).[1] |
| Oct. 28, 2000 | Possible false change back to Standard Time. |
| Oct. 29, 2000 | First return to Standard Time after end of year. |
| Dec. 29, 2000 | Last workday in year 2000.[1] |
| Dec. 31, 2000 | Last day in year 2000, and the year must have 366 days.[1] |
| Jan. 1, 2001 | Tandem Computers Operating System date overflows. |
| Jan. 2, 2001 | First workday in year 2001.[1] |
| Feb. 29, 2001 | Illegal date - must not occur. |
| Sep. 8, 2001 | The UNIX operating system date overflows at 01:46:40 UTC. |
| Dec. 31, 2001 | The year must have 365 days. |
| Jan. 1, 2002 | Critical date for several types of backward calculations |
| Jan. 1, 2002 | European banks begin final phase of Euro conversion. |
| Feb. 2, 2002 | Another "purge" date (20)02/02/02 in some computer programs. |
| Feb. 29, 2002 | Illegal date - must not occur. |
| June 30, 2002 | Last day European national currencies are acceptable. |
| July 1, 2002 | First day of Euro-only transactions. |
| Jan. 1, 2003 | Burroughs Unisys A system clock overflow. |
| Feb. 29, 2003 | Illegal date - must not occur. |
| Feb. 29, 2004 | Valid date - must occur. (First leap Day in new century) |
| Dec. 31, 2004 | Last day in year 2004, and the year must have 366 days. |
| Jan. 2005 | BDS 16-bit UNIX ("Eunix I") date overflow. |
| Jan. 1, 2007 | FAA microcode "may" fail according to FAA press release. |
| Sep. 9, 2009 | Another "purge" date (09/09/09) in some computer programs. |
| Jan. 1, 2010 | ANSI C Library date calculations overflow. |
| July 19, 2004 | GPS Receiver Almanac Rollover - 256 weeks after previous rollover. |
| Jan. 1, 20x0 | Critical "windowing" dates for many computer applications |
| Dec. 21, 2012 | The end of the world according to one Mayan calendar. |
| Apr. 7, 2019 | GPS Receiver Almanac rollover again. |
| Jan. 1, 2020 | Apple MacIntosh System 6.0.4+ date system fails. |
| Dec. 21, 2023 | The end of the world according to another Mayan calendar. |
| Jan. 1, 2026 | Quicken and Quickbooks rumored to fail. |
| Sep. 30, 2034 | Unix Computer Operating system calendar rolls over. |
| Jan. 1, 2036 | Burroughs Unisys A system date failure. |
| Feb. 6, 2036 | NT mktime timestamp overflows. |
| Jan. 1, 2037 | NTP Computer Operating system calendar rolls over. |
| Jan. 1, 2038 | Apple Mac OS X Server (Rhapsody) clock overflows. |
| Jan. 19, 2038 | Unix and Linux operating system calendar rolls over. |
| Feb. 6, 2040 | The original 128K Apple MacIntosh OS calendar fails. |
| Sep. 18, 2042 | IBM/360 Operating system calendar rolls over. |
| Jan. 1, 2044 | Older MS/PC-DOS 7-bit file date MSB fails. |
| Jan. 1, 2046 | Amiga system dates will fail. |
| Jan. 19, 2048 | Stratus VOS will fail. |
| Jan. 1, 2050 | Microsoft Project 98 explicit (4-digit) years will fail. |
| May 10, 2071 | IBM AS/400s that were reset in 2000 will fail. |
| 2072, TBA | Overflow of Milstar Satellite Operating System. |
| Jan. 1, 2079 | Microsoft Excel 95 / 7.0 explicit (4-digit) years will fail. |
| Jan. 1, 2100 | Yet another Y2k; which is NOT a leap year. |
| Feb. 28, 2100 | Valid date - must occur. |
| Feb. 29, 2100 | Illegal date - must not occur. |
| Feb. 7, 2106 | 32-bit UNIX time overflows. |
| Jan. 1, 2108 | Current Microsoft DOS / Windows 16-bit FAT tables overflow. |
Yes, some of these dates may. at first glance, appear to carry things way off the deep end. But - these are some of the small details that make our society work. And a lot of someones - especially systems architects and computer programmers - have to deal with. They deserve out thanks and support for looking so far ahead.
Sources for some of these trigger dates -
An especially informative source and discussion for some of these dates is D. Scott Secor's listing of Y2K mission-critical dates.