The magazine of the Melbourne PC User Group The Millennium Approaches Tom Coleman

How could programmers have built such a bug into our computers. The fools! 

Such ill informed codswallop is OK for the popular press who report all kinds of unlikely and romantic things about computers but hardly the kind of rumour that is going to impress a serious computer user. 

There were similar rumours abounding in 1582 when they introduced the Gregorian calendar. 

You see, they had discovered that due to small inaccuracies in the Julian Calendar, which had been in use for the previous 1600 years, the earth was losing time. These errors had accumulated to a point where the earth's orbit was seriously out of whack with what they thought it should be. 

In those days it took a Papal Decree to get things done. So Pope Gregory XIII endorsed a hiccup in the calendar to get things back on track.

His Jesuit astronomers were a bit out in their calculations and things had to be adjusted again later, resulting in the Summer and Winter Solstices being on the 21st of June and 21st of December instead of the 30th June and 31st of December as was originally intended. 

There were riots in Paris and in parts of Germany as the populace feared that by changing the calendar the church was shortening their lives. 

You thought we had problems with daylight saving. 

Some of the arguments the astronomers were trying to reconcile were the differences between the earth's rotation and its orbit. You see, the earth does not orbit the sun in an exact number of days. It takes about 365 and a quarter days. So to make a coarse adjustment we have a leap year every four years to take up the majority of the difference. The Julian Calendar got that much right. However it is not exactly a quarter of a day. There is a smidgin less.

To allow for that slight difference the Gregorian Calendar modified the way we calculate leap years. 

The turn of the century years are not Leap Years. 

This is where DOS comes into it. Actually its not DOS at all, its the date chip in your computer that DOS reads. Actually its not the chip. Its some people with a little knowledge being dangerous. 

These chips bundle the days into groups 1461 days long. These groups are divided up into 3 parts 365 days long and 1 part 366 days long. The chip then applies that to a starting date of 1-1-80 and Bob's your Uncle. 

Most of the earlier date chips did not allow for a non-Leap Year. There is no simple adjustment. However there are patches available on some BBS to make that one day adjustment. You can be sure there will be plenty more to come. 

Just when you thought it was safe to wind up your watch again we discover the awful truth. 

The year 2000 IS going to be a Leap Year. 

Another bit of fine tuning built into the Gregorian calendar says that Century years divisible by 400 are Leap Years. 

The folks who wrote those patches are going to be hopping mad. To salvage their egos I will let them write another patch. 

There will be a problem with some date arithmetic. For example, the difference between 31-12-99 and 1-1-00 is only one day, not 99 years 364 days. Some chips have difficulty coping with this. 

Most date/time chips in computers being sold today know all about the year 2000. Those few that do not will be nine year old museum pieces by then. 

Some software being sold today does its own internal date arithmetic. These will have difficulties. However the programmers have better than eight years to release a new version with a bug fix. 

If you have not updated your software by the turn of the century you will probably not get much sympathy.

Reprinted from the June 1991 issue of PC Update, the magazine of Melbourne PC User Group, Australia

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