Happy Leap Day

A search for Canadian legal decisions on “leap day” doesn’t turn up much other than an immigration case where an application for citizenship depended on the number of days the applicant was in Canada, and Leap Day became an issue in the calculation.

Some of us who dealt with the Y2K bug will recall that leap day was a potential secondary problem for the year 2000. Common wisdom is that a leap year is any year divisible by 4. But that’s not the entire formula. A leap year is any year divisible by 4, except those that are divisible by 100 unless it is divisible by 400.

The concern for the year 2000 was that if a coder knew the “except by 100” rule, but not the “by 400” exception to that rule, then the system would not have thought there was a leap day that year. Perhaps it was a good thing that any coder who just knew the “divisible by 4” part of the rule just happened to get the right result.

As it turns out, solar time is not a terribly accurate way to measure time. In addition to leap years, we add leap seconds every few years, as determined by the International Earth Rotation and Reference Systems Service. The next leap second will be on June 30, 2012 – so start planning for the extra time now. 

Leap seconds are controversial, and there is a movement to adjust in another fashion. Adding leap seconds makes life complicated for computers or anything requiring precise time. To avoid the issue, GPS, for example, runs on its own time, which is a few seconds off of UTC, or Universal Coordinated Time.


  1. At one of my former employers, we had several labour arbitrations involving leap days – salaried people working the 29th claiming overtime because of the extra working hours in February.

    We also had arbitrations involving the time change – hourly staff working the overnight shift during the “jump ahead” getting shortchanged an hour, or during the “jump back” claiming double time for the extra hour.