This is an archived page and is no longer updated.
Current content of Prof. Ladkin's research group is available at

The Year 2000 Problem

Heiko Holtkamp, Peter B. Ladkin


"Houston, we have a problem."

We describe the Year 2000 problem (the millennium bug, Y2K), what can be done about it, and what systems it is likely to affect. These pages are intended for novices as well as experts, with short descriptions of the computer-technical background linked where necessary.
These pages accompany the Bielefeld seminar "Das Y2K-Problem". We are reviewing and developing strategies for handling the Year 2000 problem. We shall be continuing to build these pages.


Back to Contents

What is the Year 2000 problem?

The Year 2000 problem arises from representing years in dates with only two digits: 27.04.98 instead of 27.04.1998. This leads to date ambiguity: is 01.01.00 the 1st January 1900, or the 1st January 2000? People using such date representations usually have no problem with this ambiguity, since one usually has enough background information to figure out which is meant. However, it is a problem for Computer programs, chips and other digital hardware that have not been specifically designed to handle this ambiguity, because computer systems work rigidly - they can't use `background information' or identify `ambiguity', unless this is specifically built in to their design.

A more precise description of the problem is that it is one example of a date representation discontinuity (DRD). The features of a DRD are that

A DRD is an example of a data discontinuity problem, itself in turn an example of data ambiguity problem. Such problems are a consequence of how data are represented by computers. Those who don't know may consult our gentle introduction to Data Representation in Computers.

Back to Contents

Other Date Representation Discontinuity Problems

The Year 2000 problem is not the only DRD problem we need to worry about now. Many computer systems (software and hardware) will have problems earlier.

Ambiguous End-of-File Markers

For example many financial and adminstrative software, especially databases, use a date of 9.9.99 (or similar constructs such as 1.1.11, 99.99.99, 00.00.00) as end-of-file markers. This is a particularly clear example of data ambiguity being deliberately designed in, when it could have been avoided. Such systems could have problems handling dates of 9th September 1999.

32-bit Unix and C

Many Unix systems (the 32-bit ones in particular) have a DRD on 19th January 2038 when the internal time representation reaches its limit and `rolls over'. UNIX calculates dates using the number of seconds since 1.1.1970 0:00:00 Coordinated Universal Time (Greenwich Mean Time). This is stored as a 32-Bit signed integer, that will reach its maximum possible value (31 1's) on 19.01.2038 and `roll over' (to 31 0's). This issus also concerns many C and C++ programs, which use the same date representation.

GPS, the global satellite navigation system

Navigation is increasingly dependent on a satellite data system run by the United States armed forces, known as the Global Positioning System (GPS). A GPS system uses data from a number of satellites it can `see' in the sky to determine precisely where it is. It is extensively used by trucking companies to tell where their trucks are, by automobiles for `moving map' displays showing the car's position, and of course by aircraft to determine position. It is also increasingly used by commercial aircraft as a primary means of navigation when approaching to land at an airport in `instrument conditions' (when the airport cannot be seen, or only sporadically, because of clouds, darkness or both). Because ensuring clearance between the aircraft and earth-based obstacles or mountains is essential `on approach', a GPS date discontinuity could have safety-critical consequences.

GPS counts dates by counting weeks since the 5 January 1980, and stores this count in a 10-bit unsigned integer. That entails a date discontinuity between week 1024 and week 1025 since 5 Jan 1980, which will happen on 21 August 1999 (apparently at 23:59:47). GPS equipment that does not specifically handle this discontinuity may have problems at that time.

Back to Contents

Leap Year Miscalculation

Another problem with the year 2000 that is not a DRD problem occurs because many programs won't recognise that the year 2000 is a leap year. As the U.K. Health and Safety Executive says: Please pay no attention to anyone who tells you otherwise. There is a February 29th, 2000. Some programs have implemented mistaken rules for leap years.
A year is a leap year when
  • the year is divisible by 4,
  • the year is not divisible by 100,
  • but when the year is divisible by 400, it's a leap year
(see, for example, Claus Tondering: Frequently asked questions about Calendars.)

This method of calculating leap years is standardised in the European Standard EN 28601 (which is German standard DIN 5008).

Programs that implement shortsighted bugfixes (for example, shifting the `time window', say by subtracting 30 from every date), without paying attention to the boundaries of the window, will sooner or later have the same problem over again.

Briefly, specific problems with the year 2000 are:

Back to Contents

Standard Date Representations

The last point in the problem list, the lack of a standard for date representations, is not necessarily a Year 2000 problem. However, adherence to a standard would (have) solve(d) most of the Year 2000 problems.

There is such a standard. The International Standard ISO 8601 requires the representation "YYYY-MM-DD". This is prescribed by European Norm EN 28601 and German Industry Norm DIN 5008. This standard has two DRDs: At year 0 AD and at year 10,000AD. The standard does not prescribe what internal representation this shall have ( signed integer, unsigned integer, etc., but the date format itself is explicitly unsigned - AD is assumed). The DRD at 0 could be a problem for archaeologists or ancient-historians, whose databases may need to represent dates BC (that is, as a signed integer). We will leave it to the reader to determine if the DRD at 10,000AD is problematic.

Back to Contents

Finer Time Representations

There are more fine-grained representations of time than by days. If one measures time in seconds, then it can be very hard to determine in advance when DRD will occur. The reason is that Universal Coordinated Time (UTC) adds leap seconds according to need. Leap seconds are added by convention, and are dependent upon physical processes (changes in the rotation of the earth, and of the earth about the sun), that are imperfectly predictable (indeed, imperfectly understood). Thus it is not possible to determine today when leap seconds will be needed in the future. For more information, see the RISKS article A definitive clarification of time measurement in volume 19(14), 14 May 1997, by John Laverty and Peter Ladkin.

Back to Contents

What is the Problem with the Year 2000 Problem?

Many companies and computer users have realised the problem too late to perform proper checks - or haven't realised it at all yet! Any system which uses dates or a clock can be vulnerable. One of the problems is that the clock may be hidden. Here is a quick run through some issues.

High-Level-Language Software

Software written in a high-level language can be inspected to see if it explicitly uses two-digit dates. However, much software is written without a precise specification of all data structures it uses. It may assume such date representations, without any overt indication that it is designed around such a representation. In this case, one hopes that testing the software when the system clock has been set forward to near midnight on 31.12.1999 will produce observable effects. However, there can be effects which are not immediately observable, but which only appear later, say when certain transactions occur. For example, a financial loan system calculates at the end of the year 2000 that you owe it, not one year interest, but a backlog of 101 years interest, and sends you the bill early in 2001.

Real-Time Systems

Real-time systems are dependent on a system clock. After precautions have been taken to limit the possibility of adverse consequences, the system can be taken off-line, the system clock can be set forward to mear midnight on 31.12.1999, and the system restarted. However, as above, there can be certain effects which only become apparent after a considerable time. For example, there could be maintenance software which `wakes up' every 24 hours, looks at the record of system behavior, and calculates what and where maintenance needs to be performed. When this software wakes up at the end of the first day of the new millennium, it may calculate some values mistakenly, based on mistaken date representations in sensors and other parts of the system, calculate that something untoward has gone on, and shuts the whole system down in emergency mode. Such behavior may not be seen unless a `roll-over' test is allowed to run for more than 24 hours.

Hidden Clocks

Even more difficult to detect are those cases in which the processor or underlying hardware has a clock, whose presence the system never makes use of, and has no access to, but which is nevertheless there, and on which the underlying hardware depends for its correct operation. One can determine the presence of such clocks - and thus such Year 2000 vulnerabilities - most appropriately by asking the original manufacturer whether one is present. This is not always easy, especially for those systems whose manufacturers no longer support them (so-called `legacy systems'). In these cases, one must do the best one can to find out for oneself whether there are such dependencies, whether based on old records of any sort, or on tests.

Safety-Critical Systems

Safety-critical systems require particular care when being taken off-line for tests, or when being tested. A good guide to the analysis and testing of such systems for Year 2000 compliance is the series of two books, Safety and the Year 2000 and Testing Safety-Related Control Systems for Year-2000 Compliance, available from the electronic bookstore of the publisher, the UK Health and Safety Executive (HSE).

Legacy Systems: An Example

An example of legacy systems which definitely have clocks is the FAA's En-Route Air Traffic Display Systems, based on IBM 9020E machines (more than 30 years old) or Raytheon 760 machines (more than 25 years old). These systems have been deemed Year 2000 noncompliant by the original manufacturers. They may be replaced by the new DSR system before the millennium end.

PC-based Systems

Perhaps the most common example of legacy systems, albeit ones whose software is understood to a much larger extent than most, are older PCs running versions of Windows 3.1 or earlier. In contrast to later versions of Windows, Windows 3.1 is applications software, which runs under the operating system MSDOS (later versions of Windows run as the operating system, mostly replacing the function of MSDOS). If you have one of these systems, be sure to take Year 2000 precautions. There may be Year 2000 dependencies in the BIOS, but happily these are changeable by the user. However, old PC software may no longer be supported by the company that programmed it, if indeed that company is still in business. You may simply be advised that that software is or could be Y2K-vulnerable, and to buy the updated version (if there is one). However, your hardware may not be supported by the new software version.

Relying on System Vendors

If one can rely on hardware and software vendors to give accurate information about the Y2K-compliance of their products, then the job of determining Y2K compliance of one's system is made much easier. However, how does one tell whether to trust the vendors? Much may depend on the vendor's reputation, and record for addressing Y2K problems, and also on the risk level of one's system application. A hospital life-support system should probably be thoroughly analysed for Y2K compliance, no matter how reputable the hardware and software manufacturers. But one can probably just wait and see if a video game machine is Y2K compliant.

A Curious Response

One of the most curious replies to queries about Y2K problems is reputed to have been given by a Thai businessman: they don't have a problem because they don't use the Julian calendar (International Herald Tribune, "Distracted Asia Ignores The Millennium Bug" by Thomas Crampton, 17-18.10.1998, p1. This anecdote is attributed to Iain Anderson, a British government advisor on millennium issues, on p16, article `No hiding place', of The Economist's survey of the millennium bug, Time Runs Out, 19 September 1998). We leave it to the reader to determine the fallacy in this reasoning.

Back to Contents

Consequences of Y2K, and DDP Avoidance

No one has determined how many resources will be used up dealing with the Millennium Bug. Estimates are many billions of dollars worldwide. Neither is anyone able to determine the extent of the problem. Estimates from computer professionals have ranged from to

Economic Consequences

One thing is clear - the millennium bug is in either case a resource hog. If the bug has been overestimated as a technical problem, then billions of dollars worth of human and other resources will have been spent chasing a will-o'-the-wisp. And if has been underestimated, then those who have not analysed their systems adequately in preparation for it will suffer greatly - and this suffering may be passed on to the general public. Either way, it is a human and economic problem of significant size, said by The Economist to be the most expensive single technological failure, in constant dollars, of all time.

Given the published experience of those who have diligently searched for Y2K problems in their systems, we judge it more likely that the millennium bug has been technically underestimated than that it has been overestimated (cf. the quote from Thomas, above).

Avoiding Such Problems in the Future

Although one will simply have to deal with whatever the consequences of the millennium bug will be, avoiding data discontinuity problems in computer systems is straightforward. Data types, including ranges, must be explicitly specified either in the requirements or when designing a computer system; and it should be determined that overflows would be explicitly announced and correctly handled. (Partial lack of such measures accounted recently for the Ariane Flight 501 failure: see The Ariane 5 Accident: A Programming Problem? by Peter Ladkin, Research Report RVS-J-98-02, and An Analysis of the Ariane 5 Flight 501 Failure - A System Engineering Perspective by Gérard Le Lann, IEEE Symposium and Workshop in Engineering of Computer-Based systems, 1997.)

Such measures are a routine part of a normal rigorous specification process. Such a specification process is adequate if it is possible to prove, formally or informally, that the design of the system fulfils its requirements, and if the coding of the system fulfils its design specification, and if the hardware correctly executes the programs according to the semantics of the language in which the code is written.

Software engineers experienced in the use of formal methods have been advocating and using such procedures, especially for safety-critical and other mission-critical systems, for decades. Such methods have by and large been used in the design of security-critical systems, and are beginning to be applied more widely for safety-critical system development, although some domains (military avionics systems) are more advanced in this respect than others (hospital health care apparatus). The emergence of the millennium bug may serve to give momentum to this effort. It provides a significant argument for rigorous specification, requirements and design analysis either on economic grounds (more formal development would have saved most of the resources used later for post hoc analysis of Y2K vulnerabilities), or on grounds of the dire consequences of the millennium bug. It seems prudent at this point to hope for the former but plan for the latter.

Back to Contents

Information about Year 2000 Problems

Martyn Thomas has written some short but splendid articles attempting to categorise the extent of Year 2000 problems. We include them below by permission and with gratitude.

Peter de Jaeger has collected and linked a vast amount of information concerning Y2K, and wrote already in September 1993 that time was running out to `solve' the Year 2000 problem (for example, in a quote from his article Doomsday 2000).

Back to Contents