Leap Years And The Millennium

By Ed O'Connell

Originally published in EUG #29

Seasons' greetings!

What's all this about 2000 not being a leap year? It is a leap year. The rules are straightforward as to which years do and do not leap.

Calling the figures of a year CCYY (This in 1996 19 is CC and 96 is YY); when YY is exactly divisible by four, that is no remainder, it is a leap year. By that logic, no year that ends 00 is a leap year and there is an eight year gap between CC96 and CC04. This corrects the slight over compensation caused by an extra day every four years. However, because it overcorrects, the following amendment is applied: where YY is equal to 00 and CC is exactly divisible by four the leap year is restored. And so 2000 is a leap year but 1900 was not - and neither will 2100 be.

I suspect 10000 is not a leap year, but I don't know. And if the British Library 'said' 2000 was a leap, I expect they were misquoted.

I know this because for years I worked as an analyst for the Human Resources (Payroll, Pensions and Personnel) software function of a Building Society. Since the early Fifties, pensions administrators have had to be aware of the 'leapness' of 2000 because pensions are heavily concerned with the retirement date of the pensioner. Thus a fifty year working life begun in 1952 ends in 2002 if the pensioner was 15 in 1952.

Computerisation for these systems began during the 1970s (and still isn't complete for occupational schemes) and they are frequently updated with the constant change in legislation. The retirement date of a pension contributor is fundamental to the whole process from its onset and will dictate the final and ongoing insurance liability of the pension provider. They need to know the exact number of DAYS involved.

Similarly mortgage providers, for all the mortgages that mature on or after 1st March 2000 they need to know they get an extra days interest on the contract. Don't think banks and Building Societies will miss out on a day's extra dosh because they don't know the calendar.

Where the real systems problems arise are with time but not date-specific software. Such as PC and network operating systems. Fortunately 29/02/00 falls on a Tuesday, midweek, and so many of the timing considerations of a cityload of traffic lights, for example, will not be felt until the following Saturday - allowing Saturday afternoon and Sunday to reset for the Monday rush. Comforting or what?

Chris Chadwick



Following on Roy Warner's letter (and utility) on leap years and the year 2000, I am surprised that the British Museum would say that 2000 is not a leap year. Although the years 1900 and 1800 were not leap years, the year 2000 is (as also would the year 1600 have been if leap years were in use then).

According to Whitaker's Almanac, the rules are as follows:

  "The number of a leap year must be exactly divisible by four unless it is the last year of the century, which can only be a leap year if it is exactly divisible by 400."

The year 2000 is of course divisible by 400 and is therefore a leap year.

They use the expression "last year of the century" because the year 100 was the last year of the first century, not the first year of the second century. It follows therefore that the year 2000 is the last year of the second millennium and 2001 is the start of the third millennium, so I wonder why everybody is getting hyped up about celebrating the year 2000?

Coming back to the possibility of software problems with the year 200, I assume it must be due to applications using field sizes of less than four characters for the year, thus being left with a value of zero when they add one to 1999. The older inflexible programmes written for mainframe computers will present the worst problems, but it is open to question whether the banks and insurance companies should be carrying these dinosaurs over to the next millennium anyway.

Ed O'Connell

Obviously this is quite a contentious issue amongst out readers, although the Almanac seems quite conclusive. Ed's final point is a good one. Maybe Bill Gates has a 'new generation' computer which he intends to release in 1999 and this hype is simply an attempt to make everyone feel that they have to buy it. E.g. Dump your old models or suffer the consequences.

Now am I cynical?

Gus Donnachaidh, EUG #29