[tz] Leap year bugs

Brian Inglis Brian.Inglis at SystematicSw.ab.ca
Wed Jan 22 19:46:47 UTC 2020

On 2020-01-22 11:37, Paul.Koning at dell.com wrote:
>> On Jan 22, 2020, at 1:32 PM, Matt Johnson-Pint <mj1856 at hotmail.com> wrote:
>> Hello Time People!  Apologies in advance, as I know this is a bit off-topic being not about time zones, but I know that many of you are invested in date and time programming, and I'd appreciate some peer discussion.
>> ...
>> My question for the community is this:  Why are Y2K and Y2038 bugs such a hot topic when they are/were one-time events, but yet leap year bugs are rarely discussed even though they are recurring and can have critical impact?  People seem to remember 20 years ago, but not 4 or 8 years back.  Why?
>> Are any of you working on leap year related issues?  Have you checked your date math for leap year bugs? If not, why?
> My guess is that getting leap years right is a simple matter that any competent programmer will understand, while Y2K and Y2038 are not so obvious.
> As you said, these others are once in a lifetime issues.  It's precisely that point that makes them less understood.  Leap years have been encounteredin software for half a century or more; it is to be expected that people know how to get this right.  
> It wouldn't surprise me to see software that falsely claims 2100 is a leap year, though.  Fortunately we have a while to deal with those bugs.

Everyone thinks they understand leap years (Feb 29 every multiple of 4 years,
right?) so ignore it, including in their code.

Oxford University had a copy with translation of Pope Gregory XIII's Inter
gravissimas bull, dated AD MDLXXXI [ante diem] sexto Kalendas Martii (O.S.) 1582
February 24 (N.S.) in his 10th year as pope, with a clear statement of the
rules, that I used to deal with confused senior IT staff and executives.
A updated variation appears at


[OT: Note the date issued on the Julian Calendar: as it was not an O.S. leap
year this was not the leap day; the year changed on Lady Day March 25 (O.S.);
still observed as HM gov fiscal and taxation year end April 5 (N.S.) as they
were not giving up 11 days of taxes in 1751/2!]

Take care. Thanks, Brian Inglis, Calgary, Alberta, Canada

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