After US Y2K Czar John Koskinen later this month closes the books on computer problems posed by the millennial date change, he'll turn to the trickier but lesser issue of leap year.

Because of a series of confusing calendar caveats inscribed centuries ago, federal officials now believe that some programmers miscalculated and assumed that the year 2000 would be a leap year.

The year 2000 is a leap year. And if all programmers stuck to the general rule for leap years - that they occur in years divisible by four - most would arrive at the correct conclusion that 2000 is a leap year.

But if programmers were to get mired in the exceptions for leap years and not find their way back to the original assumption, systems would be programmed incorrectly to leave out Feb. 29.

"The leap-year problem, I thought when I started, had to be inconsequential because to do it wrong you had to know just enough information to get into trouble," said Koskinen at a briefing just after the year-2000 date change.

Specifically, there is an exception to the "divisible by four" rule - that double-zero or centuries are not leap years. If a programmer has latched onto that exception, there could be trouble.

However, there is another exception: centuries divisible by four hundred - such as the year 2000 - are leap years.

"I didn't figure there could be very many people that got it halfway right and halfway wrong," Koskinen said.

But since the leap year test was included in standard year-2000 testing procedures, federal and private-sector officials have determined that there has been a significant miscalculation of leap year.

"I think because everybody has been testing and monitoring against it, we have worked effectively in that regard. But I think it is a date that will be worth monitoring and probably will create the potential for more glitches than certainly Oct. 1, 1999 did," Koskinen said.

"But on the other hand," Koskinen continued, "I don't think it is of the same magnitude as the challenge that we are monitoring now and in the next week."